Optical fiber and production method thereof

ABSTRACT

An optical fiber, which has a zero-material dispersion wavelength equal to or greater than 2 μm, and a high nonlinear susceptibility χ 3  equal to or greater than 1×10 −12  esu, and uses tellurite glass having sufficient thermal stability for processing into a low loss fiber, employs a PCF structure or HF structure having strong confinement into a core region. This enables light to propagate at a low loss. The size and geometry of air holes formed in the core region, and the spacing between adjacent air holes make it possible to control the zero dispersion wavelength within an optical telecommunication window (1.2-1.7 μm), and to achieve large nonlinearity with a nonlinear coefficient γ equal to or greater than 500 W −1  km −1 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/537,179, filed Jun. 1, 2005, which claims priority to Japanese PatentApplication Nos. 2003-293141 filed Aug. 13, 2003, 2004-045500 filed Feb.20, 2004, 2004-202954 filed Jul. 14, 2004, 2004-207728 filed Jul. 14,2004 and PCT/JP04/11625 filed Aug. 12, 2004, the contents of which areincorporated herein by specific reference.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to a tellurite glass optical fiber and itsfabrication method, the optical fiber having its zero dispersionwavelength controlled in an optical telecommunication window and havinghigh nonlinearity. More specifically, the present invention relates toan optical fiber and its fabrication method, the optical fiber havingits zero dispersion wavelength in a 1.2-1.7 μm band, an opticaltelecommunication window, which is implemented by designing therefractive index, structure and material of the tellurite glass fiber.

2. The Relevant Technology

Recently, studies of increasing capacity of optical communicationsystems have been made because of the explosive growth in communicationdemand due to a rapid proliferation of the Internet, and to the demandfor cost reduction of optical communication systems. In addition to timedivision multiplexing transmission systems which have been studied asmeans for increasing the capacity, wavelength division multiplexing(WDM) transmission systems, which transmit signal lights with differentwavelengths by multiplexing them onto a single optical fiber, have beendeveloped and spread at an increasingly fast pace. The WDM transmissionsystems can multiplex signals with different modulation schemes, andexpand the systems using new wavelengths, thereby being able toconstruct more flexible optical communication systems.

To expand the scale and to improve the functions of a WDM transmissionnetwork more flexibly, functional optical devices such as wavelengthconversion devices, high-speed optical switches and supercontinuumlightwave sources are essential. In the development of the functionaloptical devices, nonlinear optical devices have been studied intensivelywhich utilize the nonlinear effect in optical fibers.

The amount of production of the nonlinear effect in an optical fiber isproportional to a nonlinear optical coefficient γ. The nonlinear opticalcoefficient γ has the following relationship between an effective corecross sectional area A_(eff) and a nonlinear refractive index n₂.

γ∂n₂/A_(eff)

Accordingly, to achieve a large nonlinearity, it is necessary to use anoptical material with a large nonlinear refractive index n₂ and to makeA_(eff) small. Here, the effective core cross sectional area A_(eff) isgiven by the following expression (for example, see non-patent document1).

$A_{eff} = \frac{\left( {\int_{- \infty}^{\infty}{\int_{- \infty}^{\infty}{{{F\left( {x,y} \right)}}^{2}{x}{y}}}} \right)^{2}}{\int_{- \infty}^{\infty}{\int_{- \infty}^{\infty}{{{F\left( {x,y} \right)}}^{4}{x}{y}}}}$

Many of the silica glass nonlinear optical fibers currently reportedincrease the nonlinear refractive index of the silica glass itself bydoping germanium or the like to the core to increase the nonlinearity,and decrease the effective core cross sectional area by increasing therelative refractive-index difference by doping fluorine to the cladding.In addition, to produce the nonlinear effect at high efficiency in theoptical telecommunication window, the zero dispersion wavelength of theoptical fibers must be set at 1.2 μm-1.7 μm to fulfill the phasematching conditions.

As for the silica fiber, however, its zero-material dispersionwavelength is about 1.2 μm, and it is difficult to shift thezero-material dispersion wavelength greatly by a dopant. Thus, a methodis used which brings the wavelength dispersion value at the 1.55 μm bandclose to zero by optimizing the structural parameters of the opticalfiber (see non-patent document 2, for example).

On the other hand, optical fibers called photonic crystal fiber(abbreviated to PCF from now on) or holy fiber (abbreviated to HF fromnow on) are now reported which mainly use silica glass and have many airholes formed in the longitudinal direction inside the silica fiberintentionally (see non-patent document 3, for example).

Employing the fiber structure having such air holes can provide avariety of characteristics that cannot be achieved by optical fiberswith a conventional core-cladding structure, and hence applications tooptical fibers with high nonlinearity are expected.

However, a silica based PCF or HF having a zero dispersion wavelength of1.2 μm-1.7 μm and high nonlinearity has not yet been implemented. Inaddition, although the silica glass is superior in the transparency,since its nonlinearity is not so large, it generally lengthens theinteraction length to ensure the interaction length needed for thenonlinear effect. For example, long optical fibers of several hundredmeters are used sometimes. Thus, realizing more compact nonlinearoptical devices with higher efficiency have been much needed which usesoptical materials with higher nonlinearity.

Recently, on the other hand, technology development efforts have beenconducted for applying tellurite EDFAs (Erbium-Doped Fiber Amplifiers)to an optical communication field. The tellurite refers to telluritebased glass that is predominantly composed of TeO₂. The tellurite EDFA,which consists of a erbium-doped tellurite fiber formed by doping erbiumto tellurite based glass, is an amplifier that amplifies light byguiding the light wave through the optical fiber by several tens ofmeters. Using the tellurite EDFA enables the lumped amplification of thewavelength band from 1.53 μm to 1.61 μm which is twice or more widerthan the wavelength band from 1.53 μm to 1.56 μm that can be amplifiedby a conventional silica based EDFA or fluoride EDFA (see non-patentdocument 1). Furthermore, using the tellurite EDFA enables thefabrication of amplifiers at a wavelength in the 1.6 μm band (seenon-patent document 4). Accordingly, the tellurite EDFAs attractattention to be EDFAs for future ultra large capacity WDM systems.

As shown in FIG. 1, the cross section of an optical fiber 4 for anoptical amplifier composed of conventional tellurite glass includes acircular core 1 placed at its center, a cladding 2 covering the core'ssurroundings concentrically, and a jacket 3 further cloaking thecladding's surroundings concentrically. FIG. 2 shows a refractive-indexprofile of the optical fiber 4. Assume that the difference between therefractive index of the core 1 and the refractive index of the cladding2 is Δ1, the difference between the refractive index of the core 1 andthe refractive index of the jacket 3 is Δ3, and the difference betweenthe refractive index of the cladding 2 and the refractive index of thejacket 3 is Δ2, then Δ1 is much greater than Δ2, thereby stronglyconfining the light within the core 1.

In the optical fiber 4, the core 1 is doped with a dopant so that therefractive index of the core 1 is sufficiently greater than therefractive index of the cladding 2. Thus, a light beam travels throughthe core 1 with carrying out total reflection at the interface betweenthe core 1 and cladding 2. In addition, the dispersion can be controlledto some extent by varying the refractive index of the core 1 and thediameter of core 1. However, the single mode condition is not met whenthe diameter of the core 1 is increased. This results in a multi-modeoptical fiber having a plurality of modes, which deteriorates thetransmission characteristics. In contrast, when the diameter of the core1 is decreased, matching of connection with other devices cannot bemade. For these reasons, it is impossible for the conventional telluriteglass optical fiber to establish the control range of the dispersion.

Since the tellurite glass has large third order nonlinearity (see,non-patent document 5), it is expected to apply the tellurite glass tosuch as pulse compression, optical parametric amplification (OPA), andthird harmonic generation (THG). Here, the wavelength at which thematerial dispersion value of the tellurite glass becomes zero is locatedat a wavelength band longer than 2 μm. The wavelength dispersion valueof a high NA (Numerical Aperture) fiber used for an optical amplifier at1.55 μm band is usually of the order of −100 ps/km/nm. Accordingly, thewavelength dispersion value becomes a large value of the order of −1ps/nm even when a short optical fiber of about 10 m is used.

To apply the optical fiber to a long distance, or to high-speedwavelength division multiplexing transmission, it is necessary to bringthe wavelength dispersion value of the optical fiber as close to zero aspossible. In contrast, the zero dispersion wavelength of the telluriteglass optical fiber is at the wavelength band beyond 2 μm as mentionedabove. Accordingly, the tellurite glass optical fiber cannot make thewavelength dispersion value zero at the 1.55 μm band even if the optimumtechnique based on the well-known structural dispersion is used which isapplied to silica fibers.

Therefore it is difficult to implement the foregoing application in thepresent optical fiber telecommunication window by utilizing the highnonlinearity of the tellurite glass.

The above-mentioned PCF (or HF) is divided into two types according tothe waveguide principle. One of them is a photonic bandgap PCF thatconfines a light beam by a photonic bandgap. The PCF has a structureincluding a periodic air hole disposition and a uniform air hole size.The other of them is a refractive index waveguide PCF that confines alight beam by the total reflection achieved by effective refractiveindex of a medium having air holes. The refractive index waveguide PCFhas a structure that does not necessarily have the periodic air holedisposition or the uniform air hole size.

Such PCF or HF can make the refractive index difference greater than theconventional optical fiber by an order of magnitude, thereby being ableto achieve large structural dispersion. Because of the structuraldispersion, the silica based PCF or HF has its zero dispersionwavelength shifted to a shorter wavelength side. M. J. Gander et al.empirically measured dispersion characteristics of a silica glassoptical fiber consisting of a core without air holes and a claddinghaving air holes disposed hexagonally, and disclosed the results in thenon-patent document 6. According to the document, the dispersion valueat the 813 nm band was about −77 ps/km/nm. In addition, Birks et al.calculate the dispersion of a PCF, an optical fiber composed of a singlematerial, and advocate the effect of the dispersion compensation of thePCF in the non-patent document 7. Thus, the PCF structure or HFstructure is expected to be one of the dispersion compensation methodsof tellurite glass optical fibers.

N. G. R. Broderick et al. disclosed fibers with a PCF structure or HFstructure using multi-component glass in the patent document 1. Thedocument refers to the tellurite glass as an example of themulti-component glass, and shows that it is a composition of componentsselected from Na₂O, Li₂O, Al₂O₃, CaO, Ga₂O₃, GeO₂, As₂O₃, SrO₂, Y₂O₃,Sb₂O₅, In₂O₃, ZnO, BaO, La₂O₃, TeO₂ and TiO₂. However, the patentdocument 1 does not refer to the thermal stability or nonlinearcharacteristics of the glass or to the dispersion of the telluritefiber.

E. S. Hu et al. designed a PCF structure or HF structure using thetellurite glass, and disclosed fibers that shift the zero dispersionwavelength to 1.55 μm in the non-patent document 8. The documentdiscloses that three different PCF structures or HF structures wereformed using tellurite glass with a zero-material dispersion wavelengthof 1.7 μm, and that each structure was able to shift the zero dispersionwavelength to 1.55 μm. As for the fibers disclosed in the non-patentdocument 8, however, since the tellurite glass used have low nonlinearsusceptibility, and the zero-material dispersion wavelength is 1.7 μm,the optical confinement within the core region is insufficient, andhence it is impossible to obtain sufficiently large nonlinearity (thenonlinear coefficient γ reported was 260 W⁻¹ km⁻¹ at the maximum).

The tellurite glass has large third order nonlinearity. Accordingly,systems utilizing optical fibers composed of the tellurite glass havingthe high nonlinearity have been studied. For example, as shown in FIG.3, it has been proposed to utilize an optical fiber 8, which has a core5 and a cladding 6 composed of tellurite glass, for opticalamplification such as a Raman amplifier (see non-patent document 9, forexample).

In addition, the limit at which the gain is achieved on the longerwavelength side of the tellurite EDFA is increased by 7-9 nm comparedwith a silica based EDFA or fluoride EDFA. This enables an amplifier ata 1.6 μm band wavelength which cannot be utilized conventionally (seenon-patent document 4, for example). Consequently, the tellurite EDFAsattract attention as EDFAs in the future super large capacity WDMtransmission systems.

Fibers using the tellurite glass have been applied to Er³⁺-doped fiberamplifiers or Raman amplifiers, and implement wideband amplifiers (seenon-patent document 1 and non-patent document 8). The tellurite glasshas nonlinear effect 10 or more times greater than that of the silicaglass, and at the same time implements low loss fibers with a loss of 20dB/km in the application to the Raman amplifier. Thus, the telluriteglass has wideband optical amplification characteristics and hightransparency. In addition, the tellurite glass has large opticalnonlinear susceptibility χ³ (see non-patent document 5, for example).Accordingly, nonlinear devices are expected which are more compact andhave higher efficiency than ever.

However, it is difficult for the tellurite glass optical fibers tosatisfy the phase matching condition between the pumping light and the1.55 μm band signal light, which is the optical telecommunicationwindow, because the wavelength at which the material dispersion becomeszero is located in a wavelength band longer than 2 μm, thereby making itdifficult to utilize the nonlinearity positively. For example, thetellurite glass optical fibers used for optical amplifiers have awavelength dispersion value of about −100 ps/km/nm at the wavelength1.55 μm.

A dispersion-shifted optical fiber or dispersion compensation opticalfiber controls the dispersion by increasing the relativerefractive-index difference between the core and the cladding byapplying the structure of the conventional optical fiber. Applying themethod to the tellurite glass optical fiber, however, causes the zerodispersion wavelength to be further shifted to a longer wavelength side.Accordingly, it is very difficult for the tellurite glass optical fiberto implement the zero dispersion at the 1.55 μm band which is theoptical telecommunication window. As a result, a communication systemcannot be implemented which utilizes the optical fiber composed of thetellurite glass with high nonlinearity.

As for a fabrication method, an extrusion process is reported as afabrication method of a photonic crystal fiber or holy fiber composed ofoxide glass other than the silica-based glass (see non-patent document10, and non-patent document 11). The extrusion process fabricates apreform having air holes by heating fabricated bulk glass to a hightemperature at which it has deformable viscosity, and by pressing itinto a mold, followed by extruding it. It is difficult for the extrusionprocess to fabricate a low loss fiber because the glass is kept at ahigh temperature for a long time and undergoes deformation, and hencecrystal nuclei are apt to grow in the glass. Accordingly, loss values offibers disclosed in the non-patent documents 10 and 11 each exceed 1000dB/km, and no fibers have been implemented which have a loss usable aspractical devices.

Patent document 1: EP1313676, USP2003/0161599 “Holy optical fiber ofnon-silica based glass” Southampton University.

Patent document 2: Japanese Patent Application Laid-open No.2003-149464.

Patent document 3: Japanese Patent Application Laid-open No.2000-356719.

Non-patent document 1: A. Mori, Y. Ohishi, M. Yamada, H. Ono, Y.Nishida, K. Oikawa, and S. Sudo, “1.5 μm broadband amplifier bytellurite-based DFAs”, in OFC'97, 1997, Paper PD1.

Non-patent document 2: Shojiro Kawakami, Kazuo Shiraishi, and MasaharuOohashi, “Optical fiber and fiber mold devices”, Baifuukan, Inc. p. 97.

Non-patent document 3: A. Bjarklev, et al., “Photo Crystal Fibers TheState of The Art”, Holy fibers Symposium vol. 1.1, ECOC2002.

Non-patent document 4: A. Mori, Y. Ohishi, M. Yamada, H. Ono and S.Sudo, “Broadband amplification characteristics of tellurite-basedEDFAs”, in ECOC'97, vol. 3, 1997, Paper We2C.4, pp. 135-138.

Non-patent document 5: S. Kim, T. Yoko and S. Sakka, “Linear andNonlinear Optical Properties of TeO₂ Glass”, J. Am. Ceram. Soc., Vol.76, No. 10, pp. 2486-2490, 1993.

Non-patent document 6: M. J. Gander, R. McBride, J. D. C. Jones, D.Mogilevtsev, T. A. Birks, J. C. Knigth, and P. St. J. Russell,“Experimental measurement of group velocity dispersion in photoniccrystal fibre”, Electron. Lett., January 1999, vol. 35, no. 1, pp.63-64.

Non-patent document 7: T. A. Birks, D. Mogilevtsev, J. C. Knight, P. St.J. Russell, “Endlessly single-mode photonic crystal fiber-Dispersioncompensation using single-material fibers” Opt. Lett. 22, 1997, pp.961-963.

Non-patent document 8: ECOC2002 nonlinearity-Parametric Amplifiers 3.2.3“Design of Highly-Nonlinear tellurite fibers with Zero Dispersion Near1550 nm” Stanford University.

Non-patent document 9: “Journal of Lightwave Technology”, 2003, Vol. 21,No. 5, pp. 1300-1306.

Non-patent document 10: P. Petropoulos, et al.,“Soliton-self-frequency-shift effects and pulse compression in ananomalously dispersive high nonlinearity lead silicate holy fiber”,PD3-1, OFC2003.

Non-patent document 11: V. V. Ravi Kanth Kunth, et al., “Tellurite glassphotonic crystal fiber” PD3 ECOC2003.

Non-patent document 12: Gorachand Ghosh, “Sellmeier Coefficients andChromatic Dispersions Dispersiond for Some Tellurite Glasses”, J. Am.Soc., 78(10) 2828-2830, 1995.

Non-patent document 13: “Photonics Technology Letters”, 1999, Vol. 11,No. 6, pp. 674-676.

Non-patent document 14: A. Mori, et al., “Ultra-wideband tellurite-BasedRaman fibre amplifier”, Electronics Letter vol. 37, No. 24, pp.1442-1443, 2001.

Non-patent document 15: Govind P. Agrawal, “Nonlinear Fiber Optics”, 2ndedition, Academic Press, pp. 42-43.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify the above and other advantages and features of thepresent invention, a more particular description of the invention willbe rendered by reference to specific embodiments thereof which areillustrated in the appended drawings. It is appreciated that thesedrawings depict only illustrated embodiments of the invention and aretherefore not to be considered limiting of its scope. The invention willbe described and explained with additional specificity and detailthrough the use of the accompanying drawings in which:

FIG. 1 is a cross-sectional view showing a conventional tellurite glassoptical fiber;

FIG. 2 is a refractive-index profile of the optical fiber as shown inFIG. 1;

FIG. 3 is a cross-sectional view in a radial direction showing aschematic structure of a conventional optical fiber;

FIG. 4 is a cross-sectional view showing an optical fiber of an example1 in accordance with the present invention;

FIG. 5 is a graph illustrating a zero dispersion region in the opticalfiber of the example 1 in accordance with the present invention;

FIG. 6 is a graph illustrating wavelength dispersion characteristics ofthe optical fiber of the example 1 in accordance with the presentinvention;

FIG. 7 is an electric field distribution diagram illustrating a state ofthe electric field around the core in the optical fiber of the example 1in accordance with the present invention;

FIG. 8A is a cross-sectional view showing an optical fiber of an example2 in accordance with the present invention;

FIG. 8B is an enlarged view of a major portion of FIG. 8A;

FIG. 9 is a graph illustrating wavelength dispersion characteristics ofthe optical fiber of the example 2 in accordance with the presentinvention;

FIG. 10A is a cross-sectional view showing an optical fiber of anexample 3 in accordance with the present invention;

FIG. 10B is an enlarged view of a major portion of FIG. 10A;

FIG. 11A is a cross-sectional view showing an optical fiber of anexample 4 in accordance with the present invention;

FIG. 11B is an enlarged view of a major portion of FIG. 11A;

FIG. 12 is a cross-sectional view showing an optical fiber of an example5 in accordance with the present invention;

FIG. 13 is a cross-sectional view around the core of an optical fiber ofan example 6 in accordance with the present invention;

FIG. 14 is a cross-sectional view around the core of an optical fiber ofan example 7 in accordance with the present invention;

FIG. 15 is a cross-sectional view around the core of an optical fiber ofan example 8 in accordance with the present invention;

FIG. 16 is a cross-sectional view in a radial direction showing aschematic structure of an optical fiber of an example 9 in accordancewith the present invention;

FIG. 17 is a graph illustrating an equivalent refractive-index profileof the optical fiber of FIG. 16;

FIG. 18 is a graph illustrating the dispersion characteristics of theoptical fiber of FIG. 16;

FIG. 19 is a graph illustrating a refractive-index profile of theoptical fiber of FIG. 16;

FIG. 20 is a graph illustrating another refractive-index profile of theconventional optical fiber;

FIG. 21 is a graph illustrating another refractive-index profile of theconventional optical fiber;

FIG. 22 is a cross-sectional view in a radial direction showing aschematic structure of an optical fiber of an example 10 in accordancewith the present invention;

FIG. 23 is a cross-sectional view in a radial direction showing anotherschematic structure of the optical fiber of the example 10 in accordancewith the present invention;

FIG. 24 is a graph illustrating an equivalent refractive-index profileand a refractive-index profile of an optical fiber of an example 11 inaccordance with the present invention;

FIG. 25 is a cross-sectional view in a radial direction showing aschematic structure of an optical fiber of an example 12 in accordancewith the present invention;

FIG. 26A is a process diagram showing a first step of a fabricationmethod of a photonic crystal fiber of an example 13 and example 20 inaccordance with the present invention;

FIG. 26B is a process diagram showing a second step of the fabricationmethod of the photonic crystal fiber of the example 13 and example 20 inaccordance with the present invention;

FIG. 26C is a process diagram showing a third step of the fabricationmethod of the photonic crystal fiber of the example 13 and example 20 inaccordance with the present invention;

FIG. 26D is a process diagram showing a fourth step of the fabricationmethod of the photonic crystal fiber of the example 13 and example 20 inaccordance with the present invention;

FIG. 26E is a process diagram showing a fifth step of the fabricationmethod of the photonic crystal fiber of the example 13 and example 20 inaccordance with the present invention;

FIG. 27A is a cross-sectional view showing the photonic crystal fiber ofthe example 13 in accordance with the present invention;

FIG. 27B is an enlarged view showing a major portion of FIG. 27A;

FIG. 28 is a graph illustrating the dispersion of the photonic crystalfiber of the example 13 in accordance with the present invention;

FIG. 29 is a graph illustrating relationships between the core diameterand the zero dispersion wavelength of the photonic crystal fiber of theexample 13 in accordance with the present invention;

FIG. 30 is a diagram showing a configuration of the wavelength converterof the example 13 in accordance with the present invention;

FIG. 31 is a characteristic diagram illustrating an output spectrum ofthe wavelength converter of FIG. 30;

FIG. 32A is a process diagram showing a first step of a fabricationmethod of a photonic crystal fiber of an example 15 in accordance withthe present invention;

FIG. 32B is a process diagram showing a second step of the fabricationmethod of the photonic crystal fiber of the example 15 in accordancewith the present invention;

FIG. 32C is a process diagram showing a third step of the fabricationmethod of the photonic crystal fiber of the example 15 in accordancewith the present invention;

FIG. 33 is a cross-sectional view showing the photonic crystal fiber ofthe example 15 in accordance with the present invention;

FIG. 34 is a graph illustrating a spectrum of supercontinuum lightgenerated in the photonic crystal fiber of the example 15 in accordancewith the present invention;

FIG. 35A is a process diagram showing a fabrication method of a photoniccrystal fiber of an example 16 in accordance with the present invention;

FIG. 35B is a diagram showing the photonic crystal fiber fabricated bythe process of FIG. 35A;

FIG. 36 is a diagram showing a configuration of a wavelength variablepulse light source of the example 16 in accordance with the presentinvention;

FIG. 37 is a diagram showing a configuration of a parametric opticalamplifier of the example 16 in accordance with the present invention;

FIG. 38 is a graph illustrating an output spectrum of the parametricoptical amplifier of FIG. 37;

FIG. 39A is a process diagram showing a first step of a fabricationmethod of a photonic crystal fiber of an example 17 in accordance withthe present invention;

FIG. 39B is a process diagram showing a second step of the fabricationmethod of the photonic crystal fiber of the example 17 in accordancewith the present invention;

FIG. 40A is a cross-sectional view showing the photonic crystal fiber ofthe example 17 in accordance with the present invention;

FIG. 40B is an enlarged view of a major portion of FIG. 40A;

FIG. 41A is a process diagram showing a fabrication method of a photoniccrystal fiber of an example 18 in accordance with the present invention;

FIG. 41B is a view showing a structure of a major portion of afabrication apparatus of FIG. 41A;

FIG. 41C is a view showing the photonic crystal fiber fabricated by theprocess of FIG. 41A;

FIG. 42 is a diagram showing a configuration of an optical Kerr shutterexperimental system of an example 18 in accordance with the presentinvention;

FIG. 43A is a process diagram showing a fabrication method of a photoniccrystal fiber of an example 19 in accordance with the present invention;

FIG. 43B is a view showing a structure of a major portion of afabrication apparatus of FIG. 43A;

FIG. 43C is a view showing the photonic crystal fiber fabricated by theprocess of FIG. 43A;

FIG. 44 is a diagram showing a configuration of a nonlinear fiber loopmirror of an example 19 in accordance with the present invention;

FIG. 45 is a diagram showing a configuration of a clock reproductionapparatus of the example 19 in accordance with the present invention;

FIG. 46 is a cross-sectional view showing an optical fiber of an example20 in accordance with the present invention;

FIG. 47 is a diagram illustrating opto-electric field distribution ofthe optical fiber of the example 20 in accordance with the presentinvention;

FIG. 48 is a graph illustrating wavelength dispersion of the opticalfiber of the example 20 in accordance with the present invention;

FIG. 49 is a cross-sectional view showing an optical fiber of an example21 in accordance with the present invention;

FIG. 50 is a diagram illustrating the opto-electric field distributionof the optical fiber of the example 21 in accordance with the presentinvention;

FIG. 51 is a graph illustrating the wavelength dispersion of the opticalfiber of the example 21 in accordance with the present invention;

FIG. 52 is a cross-sectional view showing an optical fiber of an example22 in accordance with the present invention;

FIG. 53 is a diagram illustrating the opto-electric field distributionof the optical fiber of the example 22 in accordance with the presentinvention;

FIG. 54 is a graph illustrating the wavelength dispersion of the opticalfiber of the example 22 in accordance with the present invention;

FIG. 55 is a cross-sectional view showing an optical fiber of an example23 in accordance with the present invention;

FIG. 56 is a diagram illustrating the opto-electric field distributionof the optical fiber of the example 23 in accordance with the presentinvention;

FIG. 57 is a graph illustrating the wavelength dispersion of the opticalfiber of the example 23 in accordance with the present invention;

FIG. 58 is a cross-sectional view showing an optical fiber of an example24 in accordance with the present invention;

FIG. 59 is a graph illustrating the wavelength dispersion of the opticalfiber of the example 24 in accordance with the present invention;

FIG. 60 is a cross-sectional view showing an optical fiber of an example25 in accordance with the present invention;

FIG. 61 is a graph illustrating the wavelength dispersion of the opticalfiber of the example 25 in accordance with the present invention;

FIG. 62 is a cross-sectional view showing an optical fiber of an example26 in accordance with the present invention;

FIG. 63 is a graph illustrating the wavelength dispersion of the opticalfiber of the example 26 in accordance with the present invention;

FIG. 64 is a cross-sectional view showing an optical fiber of an example27 in accordance with the present invention;

FIG. 65 is a graph illustrating the wavelength dispersion of the opticalfiber of the example 27 in accordance with the present invention;

FIG. 66 is a cross-sectional view showing an optical fiber of an example28 in accordance with the present invention;

FIG. 67 is an enlarged view showing a region to become the core of theoptical fiber of FIG. 66;

FIG. 68 is a graph illustrating relationships between the zerodispersion wavelength and core size of the optical fiber of the example28 in accordance with the present invention;

FIG. 69 is a cross-sectional view showing an optical fiber of an example29 in accordance with the present invention;

FIG. 70 is an enlarged view showing a region to become the core of theoptical fiber of FIG. 69;

FIG. 71 is a graph illustrating relationships between the zerodispersion wavelength and core size of the optical fiber of the example29 in accordance with the present invention;

FIG. 72 is a cross-sectional view showing an optical fiber of an example30 in accordance with the present invention;

FIG. 73 is an enlarged view showing a region to become the core of theoptical fiber of FIG. 72;

FIG. 74 is a graph illustrating relationships between the zerodispersion wavelength and core size of the optical fiber of the example30 in accordance with the present invention;

FIG. 75 is a cross-sectional view showing an optical fiber of an example31 in accordance with the present invention;

FIG. 76 is an enlarged view showing a region to become the core of theoptical fiber of FIG. 75; and

FIG. 77 is a graph illustrating relationships between the zerodispersion wavelength and core size of the optical fiber of the example31 in accordance with the present invention.

DISCLOSURE OF THE INVENTION 1. Problems to be Solved by the PresentInvention

The present invention is implemented considering the foregoing problemsin the conventional techniques. Therefore a first object of the presentinvention to provide a tellurite glass optical fiber with highnonlinearity capable of circumventing the effect of the materialdispersion, having large effect on optical signal processing utilizingthe nonlinearity, and implementing wideband zero dispersion in theoptical telecommunication window.

A second object of the present invention is to provide a fabricationmethod of a low loss, highly efficient, tellurite glass optical fiberwhose zero dispersion wavelength is controlled in the 1.2-1.7 μm bandwhich is an optical telecommunication window.

2. Means for Solving the Problems

Generally, as the nonlinear susceptibility of glass increases its value,the zero-material dispersion wavelength is shifted to a longerwavelength. The technique to shift the zero dispersion wavelength to theoptical telecommunication window using the strong confinement of the PCFstructure or HF structure is effective as an application of a nonlinearfiber.

The inventors of the present invention show that the foregoing problemsof the conventional techniques can be solved by employing a PCFstructure or HF structure having strong confinement into a core regionin an optical fiber which has a zero-material dispersion wavelengthequal to or greater than 2 μm, and a high nonlinear susceptibility χ³equal to or greater than 1×10⁻¹² esu, and uses tellurite glass havingsufficient thermal stability for processing into a low loss fiber. Morespecifically, the inventors find that the size and geometry of air holesformed in the core region, and the spacing between adjacent air holesmake it possible to control the zero dispersion wavelength within anoptical telecommunication window (1.2-1.7 μm), and to achieve largenonlinearity with a nonlinear coefficient γ equal to or greater than 500W⁻¹ km⁻¹.

To accomplish the first object of the present invention, according tothe optical fiber of a first aspect of the present invention, there isprovided an optical fiber for transmitting light used in optical fibercommunication or optical devices, wherein at least a core region of theoptical fiber is composed of tellurite glass with a zero-materialdispersion wavelength equal to or greater than 2 μm, and the opticalfiber has air holes disposed in the optical fiber in a manner thatconfines light in a center of the optical fiber, thereby controlling thezero dispersion wavelength in a 1.2-1.7 μm band.

Here, the optical fiber may further comprise a region with an area 0.1to five times πλ² at the center of the optical fiber, where λ is awavelength of the light and π is the circular constant, wherein the airholes may be disposed in an entire cross section of the optical fiberexcept for the region, or in locations surrounding the region in thecross section so that the region becomes the core for confining thelight.

The tellurite glass with the zero-material dispersion wavelength equalto or greater than 2 μm may have a composition ofTeO₂—Bi₂O₃-LO-M₂O—N₂O₃-Q₂O₅, where L is at least one of Zn, Ba and Mg, Mis at least one alkaline element selected from Li, Na, K, Rb and Cs, Nis at least one of B, La, Ga, Al and Y, and Q is at least one of P andNb, and components of the tellurite glass are

50<TeO₂<90 (mol %)

1<Bi₂O₃<30 (mol %) and

1<LO+M₂O+N₂O₃+Q₂O₅<50 (mol %).

The tellurite material glass may be doped with at least one type ofrare-earth ions selected from Ce³⁺, Pr³⁺, Nd³⁺, Pm³⁺, Sm³⁺, Eu³⁺, Tb³⁺,Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺ and Yb³⁺.

To accomplish the first object of the present invention, according tothe optical fiber of a second aspect of the present invention, theoptical fiber is composed of tellurite glass and comprises: a coreregion; a first cladding section that is formed in such a manner as toenclose the core region, and has a plurality of air holes in acircumferential direction of the core region and along an axialdirection of the core region; and a second cladding section that isformed in such a manner as to enclose the first cladding section, andhas a refractive index approximately equal to an equivalent refractiveindex of the first cladding section.

The air holes of the first cladding section may be formed at fixedintervals along the circumferential direction of the core region, or maybe formed in a multilayer fashion in a radial direction of the firstcladding section, or may be filled with a material having a refractiveindex lower than a refractive index of the second cladding section. Thecore region may have a refractive index higher than a refractive indexof a material of the first cladding section, or the central section tobecome the core may have tellurite glass, a refractive index of whichdiffers from the refractive index of the tellurite glass, embedded inthe central section.

The relative refractive-index difference between the core region and thefirst cladding section may be equal to or greater than 2%.

The central section of a region to become the core may have air holesformed in the central section.

The air holes formed in a region other than the region to become thecore may be disposed in one of triangular lattice-like, quadrilaterallattice-like, and honeycomb geometries. The air holes may have one ofgeometries of circular cylinder, elliptical prism and polygonal prism.

The number of the air holes formed in a region other than the region tobecome the core may be three, and a diameter of a region to become thecore may be 0.6-6.5 μm. The number of the air holes may be four, and thediameter of the region to become the core is 0.6-5 μm.

To accomplish the second object of the preset invention, according to afabrication method of the optical fiber of the third aspect of thepresent invention, an optical fiber uses, at least as its core material,tellurite glass (referred to as “tellurite glass specified in thepresent invention” from now on) that has the zero-material dispersionwavelength equal to or greater than 2 μm and has a composition ofTeO₂—Bi₂O₃-LO-M₂O—N₂O₃-Q₂O₅, where L is at least one of Zn, Ba and Mg, Mis at least one alkaline element selected from Li, Na, K, Rb and Cs, Nis at least one of B, La, Ga, Al and Y, and Q is at least one of P andNb, and components of the tellurite glass are

50<TeO₂<90 (mol %)

1<Bi₂O₃<30 (mol %) and

1<LO+M₂O+N₂O₃+Q₂O₅<50 (mol %), wherein the fabrication method of theoptical fiber comprises: a first process of forming a preform by castmolding tellurite glass melt into a mold having a plurality of portionsconvex on the inside of the inner wall; and a second process ofinserting the glass preform produced in the first process into a hollowcylindrical jacket tube composed of tellurite glass, and of carrying outfiber drawing under pressure with maintaining or enlarging air holes ina gap between the glass preform and the jacket tube.

To accomplish the second object of the preset invention, according to afabrication method of the optical fiber of the fourth aspect of thepresent invention, the optical fiber uses the tellurite glass specifiedin the present invention as at least the core material, and thefabrication method of the optical fiber comprises: a first process offorming a preform by cast molding tellurite glass melt into a moldhaving a plurality of portions convex on the inner wall which isconically enlarged towards a bottom of the inner wall; a second processof forming a glass preform by injecting glass melt of core glasscomposed of tellurite glass, and by suction molding the core glassconically by volume contraction of the cladding glass; and a thirdprocess of inserting the glass preform produced by the second processinto a hollow cylindrical jacket tube composed of tellurite glass, andof carrying out fiber drawing under pressure with maintaining orenlarging air holes in a gap between the glass preform and the jackettube.

To accomplish the second object of the preset invention, according to afabrication method of the optical fiber of the fifth aspect of thepresent invention, the optical fiber uses the tellurite glass specifiedin the present invention as at least the core material, and thefabrication method of the optical fiber comprises: a first process offorming a preform by cast molding tellurite glass melt into a mold thathas a plurality of portions convex on the inner wall which is conicallyenlarged towards a bottom of the inner wall, and that has a hole in thebottom of the mold; a second process of forming a glass preform byinjecting glass melt of core glass composed of tellurite glass, and bysuction molding the core glass conically by volume contraction of thecladding glass and by causing the cladding glass to flow out of thehole; and a third process of inserting the glass preform produced by thesecond process into a hollow cylindrical jacket tube composed oftellurite glass, and of carrying out fiber drawing under pressure withmaintaining or enlarging air holes in a gap between the glass preformand the jacket tube. The second process may carry out vacuum degassingthrough the hole to cause the cladding glass to flow out of the hole.

To accomplish the second object of the preset invention, according to afabrication method of the optical fiber of the sixth aspect of thepresent invention, the optical fiber uses the tellurite glass specifiedin the present invention as at least the core material, and thefabrication method of the optical fiber comprises: a first process offorming a cylindrical glass block by cast molding tellurite glass meltinto a mold; a second process of forming a glass preform having airholes by boring holes in a longitudinal direction of the glass blockformed in the first process; and a third process of inserting the glasspreform produced by the second process into a hollow cylindrical jackettube composed of tellurite glass, and of carrying out fiber drawingunder pressure with maintaining or enlarging air holes in a gap betweenthe glass preform and the jacket tube.

To accomplish the second object of the preset invention, according to afabrication method of the optical fiber of the seventh aspect of thepresent invention, the optical fiber uses the tellurite glass specifiedin the present invention as at least the core material, and thefabrication method of the optical fiber comprises: a first process offorming a preform having air holes formed by cast molding telluriteglass melt into a mold having a jig including a plurality of cylindricalrodlike pins disposed on a base inside the mold, followed by extractingthe jig; and a second process of inserting the glass preform produced inthe first process into a hollow cylindrical jacket tube composed oftellurite glass, and of carrying out fiber drawing under pressure withmaintaining or enlarging the air holes in a gap between the glasspreform and the jacket tube.

3. Advantageous Results of the Invention

According to an optical fiber of a first aspect of the invention, thereis provided an optical fiber for transmitting light used in opticalfiber communication or optical devices, wherein at least a core regionof the optical fiber is composed of tellurite glass having highnonlinearity and a zero-material dispersion wavelength equal to orgreater than 2 μm, and the optical fiber has air holes disposed in theoptical fiber in a manner that confines light in a center of the opticalfiber, thereby being able to cause the light to propagate through thecore region, to control the zero dispersion wavelength in a 1.2-1.7 μmband by structural dispersion, and to achieve a high nonlinearcoefficient. Accordingly, the optical fiber is provided which is acompact, highly efficient, nonlinear device.

The optical fiber can further comprise a region with an area 0.1 to fivetimes πλ² at the center of the optical fiber, where λ is a wavelength ofthe light and π is the circular constant, and the air holes can bedisposed in an entire cross section of the optical fiber except for thatregion, or in locations surrounding that region in the cross section sothat the light is confined in the core consisting of the region, andtransmits through the core of the optical fiber. Thus, high order modesare suppressed effectively.

In addition, selecting the composition of the tellurite glass enablesthe fabrication of the fiber that has sufficient thermal stability forthe fiber fabrication process, high nonlinear coefficient and low loss.Among the components, TeO₂ and Bi₂O₃, which are essential for achievingthe high nonlinearity, must satisfy

50<TeO₂<90 (mol %)

1<Bi₂O₃<30 (mol %)

Otherwise, it is impossible to implement quality glass having highthermal stability and good transmission characteristics. The remainingcomponents are added to make the glass thermally stable, and tofacilitate the process by reducing the viscosity.

The tellurite material glass can be doped with at least one type ofrare-earth ions selected from Ce³⁺, Pr³⁺, Nd³⁺, Pm³⁺, Sm³⁺, Eu³⁺, Tb³⁺,Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺ and Yb³⁺. This makes it possible to implementsuch characteristics as optical amplification and filtering effect byabsorption, as well as nonlinearity.

The optical fiber, which is composed of the tellurite glass, cancomprise: a core region; a first cladding section that is formed in sucha manner as to enclose the core region, and has a plurality of air holesin a circumferential direction of the core region and along an axialdirection of the core region; and a second cladding section that isformed in such a manner as to enclose the first cladding section, andhas a refractive index approximately equal to an equivalent refractiveindex of the first cladding section. Thus, it becomes a highly nonlineartellurite fiber capable of implementing broad band zero dispersion inthe optical telecommunication window. In addition, since it can reducethe number of the air holes, a low cost, highly accurate optical fibercan be fabricated with ease.

In addition, embedding, in the core region, tellurite glass with acomposition providing a refractive index higher than that of thetellurite glass of the cladding sections makes it possible for the lightpropagating through the core region to undergo total reflection at theinterface between the embedded tellurite glass with the higherrefractive index and the tellurite glass surrounding it, and topropagate through the core of optical fiber. Thus, the transmission lossof the light can be reduced.

Furthermore, filling the air holes with a material with a refractiveindex lower than the refractive index of the tellurite glass can improvethe mechanical strength of the optical fiber in its entirety. Moreover,as compared with the case where the air holes are filled with air, sincethe geometry of the air holes can be more easily maintained in theprocess of drawing the optical fiber from the preform, the fabricationquality is improved. In addition, compared with the case where the airholes are filled with air, the light scattering loss can be reduced.

In addition, the air holes can be disposed in triangular lattice-like,quadrilateral lattice-like, or honeycomb geometries. This makes itpossible for the light to concentrate in the core surrounded with theair holes, and to propagate through the core, which obviates the needfor fabricating the optical fiber at high accuracy, and hence can curbthe fabrication cost.

Furthermore, the air holes can have a geometry of circular cylinder,elliptical prism or polygonal prism. This makes it possible for thelight to concentrate in the core surrounded with the air holes, and topropagate through the core, which obviates the need for fabricating theoptical fiber at high accuracy, and hence can curb the fabrication cost.

According to the fabrication method of the optical fiber of a third toseventh aspect of the present invention, the glass is fabricated by castmolding the preform using a tellurite glass composition which has highthermal stability and has a nonlinear susceptibility (χ³) 30 or moretimes greater than that of silica glass. Since the method can reduce theduration of the heating process of the glass preform as compared withthat of the conventional extrusion process, the method enables thefabrication of the low loss tellurite fiber in volume. In addition,since it can shift the zero dispersion to the 1.2-1.7 μm band, which isthe optical telecommunication window, it can provide the optical fiberwhich is a compact, highly efficient nonlinear device.

4. Description of Reference Numerals

-   -   10 optical fiber    -   11 air holes    -   12 core    -   13 cladding    -   100, 120, 130, 140, 150, 160, 170 optical fiber    -   101, 111 core section    -   102 first cladding section    -   102 a air holes    -   103 second cladding section    -   201 metal mold    -   202 glass melt    -   203 glass preform    -   204 jacket tube    -   205 elongated preform    -   206 portion at which line diameter is constant    -   207 photonic crystal fiber    -   208 portion in which holes are formed    -   2101, 2301, 2305, 2401, 2405, 2501, 2601, 2701, 2801, 2805,        2901, 2905, 3001, 3101, 3201, 3205, 3301, 3305 tellurite glass    -   2102, 2302, 2402, 2502, 2602, 2702, 2802, 2902, 3002, 3102,        3202, 3302 region to become core    -   2103, 2303, 2403, 2503, 2505, 2603, 2703, 2803, 2903, 3003,        3103, 3203, 3303 air holes    -   2104, 2304, 2404, 2504, 2604, 2704, 2804, 2904, 3004, 3104,        3204, 3304 jacket tube

5. Best Mode for Carrying out the Invention

An embodiment in accordance with the present invention has, in aphotonic crystal fiber using tellurite glass, a structure that has aplurality of air holes with a refractive index of unity around theportion corresponding to the core, and controls the zero dispersionwavelength at the 1.2-1.7 μm band which belongs to the opticalcommunication band. In particular, it is preferable that the number ofair holes be made four, so that the portion corresponding to the core issupported by cross-shaped cladding glass. Maintaining the structuralsymmetry by forming an even number of air holes enables the reduction inthe polarization dependence. In addition, the simple structure with fourair holes can facilitate the control of the elongating process, anddividing the mold for fabricating the preform into for subdivisions canfacilitate the extraction of the glass preform.

Selecting the composition of the tellurite glass appropriately in theembodiment in accordance with the present invention makes it possible tofabricate a highly nonlinear coefficient, low loss fiber which isthermally stable enough for fiber fabrication process. Among thecomponents, although TeO₂ and Bi₂O₃ are essential to provide highnonlinearity, quality glass that has high thermal stability and goodtransmission characteristics cannot be obtained if they deviate from thefollowing ranges.

50<TeO₂<90 (mole percent)

1<Bi₂O₃<30 (mole percent)

The remaining components are doped to thermally stabilize the glass, andto facilitate the processing by reducing the viscosity.

In the embodiment in accordance with the present invention, doping atleast one of Ce³⁺, Pr³⁺, Nd³⁺, Pm³⁺, Sm³⁺, Eu³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺,Tm³⁺, and Yb³⁺ to the tellurite glass material as rare-earth ions canprovide not only the nonlinearity, but also characteristics such asoptical amplification, filtering effect due to absorption, and the like.

A tellurite fiber of the embodiment in accordance with the presentinvention will now be described. Glass that is predominantly composed ofTeO₂ has a refractive index n_(D) of about two. In addition, itsmaterial wavelength dispersion has large negative dispersion in the1.2-1.7 μm band, and the zero dispersion wavelength is located at alonger wavelength side beyond 2 μm (see non-patent document 12, forexample). Accordingly, even if a fiber with a step-index core/claddingrefractive index profile is fabricated using the tellurite glass, it isimpossible to greatly change the wavelength dispersion characteristicsof the fiber from the characteristics of the material wavelengthdispersion.

Table 1 shows an example of a glass composition (mole percentrepresentation) used for the tellurite photonic crystal fiber of theembodiment in accordance with the present invention, and test resultsmeasured for thermal stability (Tx-Tg: ° C.), refractive index n_(D),nonlinear susceptibility χ³(esu), ultraviolet absorption edge UV (nm),and zero-material dispersion wavelength (μm) of each glass composition.

TABLE 1 No 1 2 3 4 5 6 7 8 9 10 11 12 TeO₂ 75 80 70 65 55 40 60 65 65 7070 70 Bi₂O₃ 0 2 10 20 30 10 8 15 12 12 7 8 ZnO 5 5 7 5 8 BaO 5 5 20 7 5MgO 5 5 Li2O 10 10 13 10 Na2O 7 5 6 5 K2O 8 5 Rb2O 5 10 10 Cs2O 5 8 B2O35 4 La2O3 5 5 Ga2O3 6 5 2 Al2O3 7 3 Y2O3 5 5 5 P2O5 5 5 Nb2O3 10 5 Tx −Tg 90 150 120 140 70 80 150 140 180 160 150 160 nD 1.98 2.07 2.18 2.212.23 1.85 2.15 2.2 2.18 2.13 2.1 2.05 χ³ (×10⁻¹²) 0.5 1 1.2 2 2.6 0.61.2 2 1.8 1.3 1.2 1.1 UV (nm) 360 365 370 378 410 390 380 390 385 375370 380 Zero (μm) 1.8 2.1 2.4 2.7 3 1.9 2.2 2.6 2.4 2.3 2.2 2.15 No 1314 15 16 17 18 19 20 21 22 23 24 TeO₂ 70 75 75 75 75 80 80 80 80 85 8590 Bi₂O₃ 10 15 10 10 7 7 8 7 7 5 5 5 ZnO 5 5 5 3 BaO 5 5 5 MgO 5 Li2O 128 Na2O 5 5 K2O 7 7 7 Rb2O 10 5 Cs2O 5 5 B2O3 5 La2O3 5 Ga2O3 5 Al2O3Y2O3 5 5 P2O5 3 Nb2O3 8 3 Tx − Tg 170 150 300< 140 300< 300< 300< 160180 140 130 50 nD 2.09 2.2 2.09 2.03 2.07 2.12 2.11 2.13 2.08 2.12 2.132.16 χ³ (×10⁻¹²) 1.5 2 1.6 1.4 1.3 1.7 1.5 1.8 1.5 1.6 1.5 2 UV (nm) 375380 375 380 370 375 370 380 370 365 380 390 Zero (μm) 2.3 2.5 2.3 2.22.3 2.5 2.4 2.6 2.2 2.5 2.6 2.8

Glass test samples were fabricated by the following procedure. Materialswere mixed in a glove box filled with nitrogen gas, and melted in anoxygen atmosphere at 800-1100° C. using a gold or platinum crucible,followed by flowing the melt into a mold preheated at 300-400° C. Sincethe process to form a fiber requires reheating such as elongating andfiber drawing of the glass preform, the thermal stability is animportant factor to implement a low loss, strong fiber. Since thetellurite glass usually undergoes the elongating and fiber drawingprocess at a temperature higher than the glass transition temperature Tgby 30-80° C., the index of the thermal stability in terms of Tx(crystallization temperature)−Tg (glass transition temperature) ispreferably 100° C. or higher.

Among the glass compositions of Table 1, although the doped amounts ofBi₂O₃ of samples No. 1-5 vary from 0 to 30 mol %, as for the samples No.1 and No. 5 whose mole percent is 0 mol % and 30 mol %, respectively,the index of the thermal stability in terms of Tx-Tg is equal to or lessthan 100° C., which indicates that the thermal stability is not enough.The sample No. 6 includes TeO₂ of equal to or less than 50 mol %, andthe sample No. 24 includes TeO₂ of equal to or greater than 90 mol %, inwhich case Tx-Tg is equal to or less than 100° C., which indicates thatthe thermal stability is insufficient. TeO₂ and Bi₂O₃ are essentialcomponents for achieving the high nonlinearity in the presentembodiment, and they must be in the following ranges from the viewpointdescribed above.

50<TeO₂<90 (mol %)

1<Bi₂O₃<30 (mol %)

1<LO+M₂O+Q₂O₃+R₂O₅<50 (mol %)

It is seen that compositions other than the foregoing examples lack thethermal stability for processing into the fiber.

Summarizing the compositions of Table 1, the tellurite glass has acomposition of TeO₂—Bi₂O₃-LO-M₂O-Q₂O₃—R₂O₅, where L is at least one ofZn, Ba and Mg, M is at least one of Li, Na, K, Rb and Cs, Q is at leastone of B, La, Ga, Al and Y, and R is at least one of P and Nb. As forthe tellurite glass having the high nonlinearity and thermal stabilityat the same time, the wavelength at which the material dispersionbecomes zero is equal to or greater than 2 μm, which is an importantphysical property value for designing the structure for controlling thezero dispersion wavelength of the fiber.

Referring to the accompanying drawings, examples in accordance with thepresent invention will be described in detail. Although the embodimentsof the optical fiber in accordance with the present invention will bedescribed by way of examples, the present invention is not limited tothe following examples. Furthermore, although the following examples areoptical fibers having the photonic bandgap structure as the basis of thewaveguide principle, a total reflection structure based on the effectiveindex difference between the core and cladding is also prepared.Therefore the optical fiber does not necessarily require the photonicbandgap condition or periodicity/uniformity.

EXAMPLE 1

FIG. 4 shows a cross section of the optical fiber of the example 1 inaccordance with the present invention. As shown in FIG. 4, an opticalfiber 10 composed of tellurite glass whose zero-material dispersionwavelength is 2.08 μm has a lot of circular air holes 11 which arefilled with air. Accordingly, the refractive index of light in the airholes 11 is approximately equal to one, the refractive index of light ina vacuum.

As for the disposition of the air holes 11 in a cross section in thedirection of the diameter of the optical fiber 10, it has a triangularlattice-like arrangement consisting of individual vertices of a lot oftriangles placed in such a manner that they are regularly (periodically)adjacent to each other. The air holes 11 have the same structure in thelongitudinal direction of the optical fiber 10. In other words, the airholes 11 are disposed uniformly in the longitudinal direction ratherthan in the photonic bandgap manner disposed three dimensionally.Accordingly, the cross section of the optical fiber has the samestructure throughout the longitudinal direction of the optical fiber 10disregarding fluctuations (distortion) in the geometry due to thefabrication process of the optical fiber 10. Thus no structure ispresent in which the air holes intersect the longitudinal direction ofthe optical fiber 10 orthogonally or obliquely. In other words, the airholes 11 disposed in the optical fiber 10 extend continuously in thelongitudinal direction of the optical fiber 10, and have the same crosssection at any locations in the longitudinal direction.

However, at the center of the optical fiber 10, the disposition of theair holes 11 lacks periodicity. The region surrounded by the air holes11 arranged with lacking the periodicity is from 0.1 to five time ofπλ², where λ is the wavelength of the light, and π is the circularconstant. The region becomes the core 12 to which the light isconcentrated, and the light does not propagate from that region in theradial direction of the optical fiber 10. In other words, the opticalfiber 10 has a photonic bandgap structure having a diffraction gratingin which the air holes 11 are disposed periodically arrangement. Thus,the optical fiber 10 has the core 12 at the center of the optical fiber10, and the cladding 13 including the air holes 11 periodically disposedaround the core 12. Incidentally, changing the spacings between adjacentair holes makes it possible to vary the diameter of the core 12, thatis, the region surrounded by the air holes 11 arranged with lacking theperiodicity.

Assume that the spacing between adjacent air holes is A, and thediameter of the air holes is d. Then, the region which brings about thezero dispersion in the optical fiber 10 is the region B as shown in FIG.5 enclosed by the line given by connecting the points at which (Δ, d) is(0, 0) and (5, 5), and the line given by connecting the points at which(Δ, d) is (2, 0) and (5, 4). The point A, at which (Δ, d) is (2.3, 2.0),belongs to the region B in which zero dispersion is achieved.

When the material composition of the tellurite glass is changed, thewavelength at which the dispersion becomes zero varies in the range from1.3 μm to 1.6 μm. In this case, although the range of the spacing Abetween the adjacent air holes and the range of the diameter d of theair holes vary, they are substantially present in the region B as shownin FIG. 5.

FIG. 6 illustrates the wavelength dispersion characteristics at point Aof FIG. 5 of the optical fiber 10, the spacing Δ of the adjacent airholes of which is 2.3 μm, and the diameter d of the air holes of whichis 2.0 μm. As illustrated in FIG. 6, the optical fiber 10 has the zerodispersion at the wavelength 1.56 μm. FIG. 7 illustrates theopto-electric field distribution around the core of the optical fiber10, which is obtained by applying the calculus of finite differencemethod, one of the numerical calculations, to the optical fiber 10. InFIG. 7, solid lines show contours every 10% difference in the electricfield. As illustrated in FIG. 7, the optical fiber 10 has a structurethat confines the light in the core 12 as the ordinary optical fiber.

Thus, the optical fiber 10 can confine the light in the core 12 by thephotonic bandgap or total reflection effect, thereby being able tosuppress the high order modes effectively, and to maintain the singlemode condition in spite of an increase in the diameter of the core 12.

EXAMPLE 2

FIGS. 8A and 8B each show an optical fiber cross section of an example 2in accordance with the present invention. In FIGS. 8A and 8B, thereference numeral 21 designates an air hole which is filled with air andhas a refractive index of approximately one. The reference numeral 22designates tellurite glass with the same composition as the sample No.18 of Table 1. A fiber 20 of the present example has a lot of air holes21 disposed in the entire region of a cross section except for itscenter in a triangular lattice-like fashion. In addition, the followingtwo types of fibers were fabricated: A first fiber has tellurite glassembedded at the fiber central section 23, which tellurite glass has azero-material dispersion wavelength of 2.1 μm and a refractive index1.1% higher than the tellurite glass 22 in terms of the relativerefractive-index difference; and a second fiber has a region to becomethe core for transmitting light by embedding tellurite glass in thefiber central section 23, which tellurite glass has a refractive index0.5% lower than the tellurite glass 22 in terms of the relativerefractive-index difference. The two fibers each have an outsidediameter of 105 μm, an air hole diameter d of 1.6 μm, an air holespacing Λ of 2.2 μm, a tellurite glass diameter b of 1.5 μm which isembedded into the central section 23 and a core diameter a of 2.8 μmthrough which the light propagates.

The two types of fibers, which were fabricated by the extrusion process,were cut and polished, followed by observing a near field pattern (NFP)and far field pattern (FFP), thereby confirming that the light wasconfined in the fiber central section 23, and the single mode wasachieved. FIG. 9 illustrates measured results of the wavelengthdispersion of the optical fiber. The zero dispersion wavelength λ₀ ofthe present example was 1.63 μm for the first fiber into which thetellurite with the refractive index of 1.1% higher was buried, and 1.58μm for the second fiber into which the tellurite with the refractiveindex of 0.5% lower was buried as illustrated in FIG. 9. In addition,their effective core cross sectional areas A_(eff) were 3.7 μm² and 3.9μm², and their nonlinear coefficient γ values were 650 W⁻¹ km⁻¹ and 610W⁻¹ km⁻¹, respectively.

EXAMPLE 3

FIG. 10A shows an optical fiber cross section of an example 3 inaccordance with the present invention. In FIG. 10A, the referencenumeral 21 designates an air hole which is filled with air and has arefractive index of approximately one. The reference numeral 22designates tellurite glass with the same composition as the sample No.15 of Table 1. A fiber of the present example has a lot of air holes 21disposed in the entire region of a cross section except for its centerin a triangular lattice-like fashion, and a region 24 to become a corefor transmitting light is formed. The outside diameter D of the fiber is105 μm. In addition, as illustrated in FIG. 10B, the air hole diameter dis 1.2 μm, the air hole spacing A is 1.5 μm, and the diameter a of thecore for transmitting light is 1.8 μm.

The fabricated fiber was cut and polished, followed by observing a nearfield pattern (NFP) and far field pattern (FFP), thereby confirming thatthe light was confined in the fiber central section, and the single modewas achieved. The zero dispersion wavelength λ₀ of the present examplewas 1.3 μm. The core region 24, which is approximately represented byπ(a/2)², where a is the core diameter and π is the circular constant,must have an area from 0.1 to five times of πλ², where λ is thewavelength. If the area is equal to or less than 0.1 times, the modecannot be established, and at the same time, connection with a silicafiber is difficult. If the area is equal to or greater than five times,the zero dispersion becomes equal to or greater than 1.7 μm, and at thesame time, the multi-mode propagation occurs.

EXAMPLE 4

FIG. 1A shows an optical fiber cross section of an example 4 inaccordance with the present invention. In FIG. 11A, the referencenumeral 44 designates a jacket. The reference numeral 41 designates anair hole which is filled with air and has a refractive index ofapproximately one. The reference numeral 45 designates tellurite glasswhose zero-material dispersion wavelength is 2.18 μm. The fiber of thepresent example has four holes 41 disposed therein, and a region 46 tobecome a core for transmitting light. The outside diameter D of the is120 μm, and the inside diameter of the air holes is 40 μm. As for thesize of the core region, a side a of the square which is inscribed inthe core region as shown in FIG. 11B is made 2.0 μm.

The fabricated fiber was cut and polished, followed by observing a nearfield pattern (NFP) and far field pattern (FFP), thereby confirming thatthe light was confined in the fiber central section, and the single modewas achieved. The zero dispersion wavelength λ₀ of the present examplewas 1.46 μm. The core region 24, which is approximately represented byπ(a/2)², where a is the core diameter and π is the circular constant,must have an area from 0.1 to five times of πλ², where λ is thewavelength. If the area is equal to or less than 0.1 times, the modecannot be established, and at the same time, connection with a silicafiber is difficult. If the area is equal to or greater than five times,the zero dispersion becomes equal to or greater than 1.7 μm, and at thesame time, the multi-mode propagation occurs.

EXAMPLE 5

FIG. 12 shows a cross section of the optical fiber of an example 5 inaccordance with the present invention. As shown in FIG. 12, an opticalfiber 30 composed of tellurite glass whose zero-material dispersionwavelength is 2.1 μm has a lot of circular air holes 31 which aredisposed in a triangular lattice-like fashion, that is, in a periodicmanner as in the foregoing example 1. However, at the center of theoptical fiber 30, the disposition of the air holes 31 lacks periodicity.In addition, the air holes 31 are filled with a glass material whoserefractive index is lower than that of the tellurite glass 33 by Δn.Since the air holes 31 separated from the central of the optical fiber30 are arranged periodically, they form a cladding 33 for making thetotal reflection of light. In contrast, the region surrounded by the airholes 31 arranged at the center of the optical fiber 30 with lackingperiodicity forms a core 32 that guides the light. The region has anarea from 0.1 to five times of πλ², where λ is the wavelength of thelight and π is the circular constant.

According to the optical fiber 30, since the air holes 31 constitutingthe photonic gap are filled with the material whose refractive index islower than that of the tellurite glass 33, the mechanical strength ofthe optical fiber is increased in its entirety. In addition, as a resultof filling the material can facilitate keeping the geometry of the airholes 31 in the process of drawing the optical fiber 30 from the preformof the optical fiber 30 as compared with the case where the air holes 31are filled with air, thereby being able to improve the fabricationquality. Furthermore, as compared with the optical fiber whose air holes31 are filled with air, it can reduce the scattering loss of light.

EXAMPLE 6

FIG. 13 shows an optical fiber of an example 6 in accordance with thepresent invention. As shown in FIG. 13, the optical fiber 40 composed oftellurite glass changes the arrangement of the air holes 31 of theoptical fiber 30 described in the foregoing example 5. The dispositionof the air holes 41 in the optical fiber 40 is a quadrilaterallattice-like arrangement consisting of a lot of quadrilateral verticesarranged adjacently in a regular (periodical) fashion in a cross sectionin the direction of the diameter of the optical fiber 40. Thearrangement of the air holes 41, however, lacks the periodicity at thecenter of the optical fiber 40. Since the air holes 41 separated fromthe central of the optical fiber 40 are arranged periodically, they forma cladding 43 for making the total reflection of light. In contrast, theregion surrounded by the air holes 41 arranged at the center of theoptical fiber 40 with lacking periodicity forms a core 42 through whichthe light propagates. The region has an area from 0.1 to five times thearea of πλ², where λ is the wavelength of the light and π is thecircular constant. Incidentally, the air holes 41 are filled with amaterial whose refractive index is lower than that of the telluriteglass.

Thus, the optical fiber 40 can offer the same effect and advantages asthe optical fiber 30 described in the foregoing example 5.

As for the air holes 41, they can be simple air holes filled with air.In either case, the light is confined in the core 42, and the high ordermode can be suppressed effectively. Thus, the single mode condition canbe maintained in spite of an increase in the diameter of the core 42.

EXAMPLE 7

FIG. 14 shows an optical fiber of an example 7 in accordance with thepresent invention. As shown in FIG. 14, the optical fiber 50 composed oftellurite glass changes the arrangement of the air holes 41 of theoptical fiber 40 described in the foregoing example 6. The air holes 51in the optical fiber 50 are disposed at vertices of a hexagonal(honeycomb), which are arranged adjacently in a regular (periodical)fashion in a cross section in the direction of the diameter of theoptical fiber 50. The arrangement of the air holes 51, however, lacksthe periodicity at the center of the optical fiber 50. Since the airholes 51 separated from the central of the optical fiber 50 are arrangedperiodically, they form a cladding 53 for making the total reflection oflight. In contrast, the region surrounded by the air holes 51 arrangedat the center of the optical fiber 50 with lacking periodicity forms acore 52 through which the light propagates. The region has an area from0.1 to five times of πλ², where λ is the wavelength of the light and πis the circular constant. Incidentally, the air holes 51 are filled witha material whose refractive index is lower than that of the telluriteglass.

Thus, the optical fiber 50 can offer the same effect and advantages asthe optical fiber 40 described in the foregoing example 6.

As for the air holes 51, they can be simple air holes filled with air.In either case, the light is confined in the core 52, and the high ordermode can be suppressed effectively. Thus, the single mode condition canbe maintained in spite of an increase in the diameter of the core 52.

EXAMPLE 8

FIG. 15 shows an optical fiber of an example 8 in accordance with thepresent invention. As shown in FIG. 15, the optical fiber 60 composed oftellurite glass changes the arrangement of the air holes 31 of theoptical fiber 30 described in the foregoing example 5. In the opticalfiber 60, a lot of air holes 61 have a hexagonal shape in a crosssection perpendicular to the longitudinal direction of the optical fiber60. The arrangement of the air holes 61, however, lacks the periodicityat the center of the optical fiber 60. Since the air holes 61 separatedfrom the central of the optical fiber 60 are arranged periodically, theyform a cladding 63 for making the total reflection of light. Incontrast, the region surrounded by the air holes 61 arranged at thecenter of the optical fiber 60 with lacking periodicity forms a core 62through which the light propagates. The region has an area from 0.1 tofive times of πλ² where λ is the wavelength of the light and π is thecircular constant. Incidentally, the air holes 61 are filled with amaterial whose refractive index is lower than that of the telluriteglass.

Thus, the optical fiber 60 can offer the same effect and advantages asthe optical fiber 30 described in the foregoing example 5.

As for the air holes 61, they can be simple air holes filled with air.In either case, the light is confined in the core 62, and the high ordermode can be suppressed effectively. Thus, the single mode condition canbe maintained in spite of an increase in the diameter of the core 62.

As for the arrangement of the air holes forming the diffraction gratingof the photonic crystals constituting the photonic bandgap, it is notlimited in particular as long as it can confine the light in the core sothat the light does not propagate in the radial direction from thecenter of the optical fiber, and it has a periodic disposition, that is,regular lattice-like disposition.

As for the shape of the air holes, it is not limited to a circularcylinder (circular air hole), but it may be a shape of a triangularprism (triangular air hole), rectangular prism (rectangular air hole),or hexagonal prism (hexagonal air hole), any of which can implement thewaveguide structure based on the photonic bandgap.

EXAMPLE 9

Next, an optical fiber of an example 9 in accordance with the presentinvention will be described with reference to FIGS. 16-19. In this case,as the tellurite glass is used glass as defined in claim 1, which hasthe zero dispersion wavelength at 2 μm or above. In particular, amongthe glass composition ratios in the foregoing Table 1, the compositionratios other than No. 1 and No. 6 are effective. In addition, the glassmaterial as defined in claim 3, which is doped with rare-earth is alsoeffective. FIG. 16 is a cross-sectional view in a radial directionshowing a schematic structure of the optical fiber; FIG. 17 is a graphillustrating an equivalent refractive-index profile of the optical fiberof FIG. 16; FIG. 18 is a graph illustrating the dispersioncharacteristics of the optical fiber of FIG. 16; and FIG. 19 is a graphillustrating a refractive-index profile of the optical fiber of FIG. 16.Incidentally, the term “equivalent refractive index” in the presentinvention refers to a refractive index that acts on the lightsubstantially.

The optical fiber of the example 9 in accordance with the presentinvention is an optical fiber 100 composed of tellurite glass as shownin FIG. 16. It includes a core section 101; a first cladding section 102that is formed in such a manner as to enclose the core section 101, andhas a plurality of circular air holes 102 a along the axial direction ofthe core section 101 in the circumferential direction of the coresection 101; and a second cladding section 103 that is formed in such amanner to enclose the first cladding section 102, and has a refractiveindex approximately equal to the equivalent refractive index of thefirst cladding section 102.

The first cladding section 102 has a plurality of (six in the presentexample) air holes 102 a formed at regular spacings along thecircumferential direction of the core section 101. The air holes 102 aof the first cladding section 102 are filled with air whose refractiveindex is approximately one equal to the refractive index in a vacuum.The relative refractive-index difference (Δ) between the refractiveindex of the core section 101 and the equivalent refractive index of thefirst cladding section 102 is equal to or greater than 2%. The secondcladding section 103, using tellurite glass with a composition differentfrom that of the tellurite glass of the core section 101, has arefractive index lower than the refractive index of the core section101, and approximately equal to the equivalent refractive index of thefirst cladding section 102.

The optical fiber 100 of the example 9 in accordance with the presentinvention is designed such that the radius r of the air holes 102 a is0.5-1.0 μm, the pitch γ between the air holes 102 a is 1.0-2.0 μm, andthe radius rr of the first cladding section 102 is equal to or less than3 μm.

Incidentally, since the optical fiber 100 maintains a uniform structurein the axial direction, the cross section structure in a radialdirection is the same throughout the length in the axial directiondisregarding the fluctuations in the geometry due to the fabricationprocess. Thus no structure is present which intersects the axialdirection of the optical fiber 10 orthogonally or obliquely.

In the optical fiber 100 of the example 9 in accordance with the presentinvention, the air holes 102 a are disposed singly on the vertices of aregular hexagon to form the first cladding section 102. Since the airholes 102 a are not formed in the core section 101 at the center, thecore section 101 has the highest refractive index, thereby concentratinglight to the core section 101.

As for the optical fiber 100 of the example 9 in accordance with thepresent invention, it is found that the polarity is reversed at the zerodispersion wavelength as illustrated in FIG. 18, and the dispersionbecomes flat in a particular wavelength region. Accordingly, the opticalfiber 100 of the example 9 in accordance with the present invention canimplement a broad zero dispersion wavelength region.

As described in the section of the background art, optical fibers calledphotonic crystal fibers (PCFs) or holy fibers (TFs) have been developedrecently which use silica glass and have air holes formed intentionally.The PCFs or TFs are divided into two types according to the waveguideprinciple. One of them is a photonic bandgap type that confines light bythe photonic bandgap, and its structure requires strict periodicity anduniformity of the air hole size. The other of them is a refractive indexwaveguide type that confines light by the total reflection achieved bythe effective refractive index difference of the medium having the airholes, and its structure does not necessarily requires the strictperiodicity or the uniformity of the air hole size.

For example, the foregoing non-patent document 6 reports experimentallymeasured results of the dispersion characteristics of a silica glassoptical fiber including a core section without air holes and a claddingsection having air holes arranged in a hexagonal fashion. The non-patentdocument 6 reports the optical fiber that has a dispersion value ofabout −77 ps/km/nm at the wavelength of 813 nm. In addition, thenon-patent document 13, for example, calculates the dispersion of anoptical fiber (PCF) composed of a single material, and reports thedispersion compensation effect of the PCF.

Thus, the inventors of the present invention have concentrated all theirenergies on the study, and found that the foregoing problems can besolved by providing tellurite glass optical fibers with a PCF or HFstructure. More specifically, the inventors of the present inventionconfirmed that it was possible to implement broad band zero dispersionwavelength in the 1.55 μm band which was the optical telecommunicationwindow, and to implement a highly nonlinear optical fiber 100 with highoptical confinement effect by making the relative refractive-indexdifference (Δ) between the refractive index of the core section 101without the air holes 102 a and the equivalent refractive index of thefirst cladding section 102 equal to or greater than 2%, and byapproximately matching the equivalent refractive index of the firstcladding section 102 with the refractive index of the second claddingsection 103. In addition, the inventors of the present invention foundthat the zero dispersion wavelength and the optical confinement effectwere controllable in a wide range by the size and spacing of the airholes 102 a. Furthermore, the inventors of the present invention made itpossible to implement the low refractive index by using, as the secondcladding section 103, tellurite glass with the composition differentfrom that of the tellurite glass used as the core section 101. Inaddition, the inventors of the present invention made it possible tofabricate the optical fiber at low cost without forming air holes in thesecond cladding section 103.

Incidentally, the patent document 2 proposes a dispersion compensationtype having the wavelength dispersion equal to or greater than +80ps/nm/km at wavelengths from 1400 to 1800 nm by forming air holes 122 ain a silica glass optical fiber 120 with a core section 121 and acladding section 122 as shown in FIG. 20. The optical fiber 120 has thediameter of the core 121 increased to about 20 μm to reduce theoptically nonlinear characteristics, and has a low A structure with therelative refractive-index difference (Δ) between the core 121 andcladding 122 equal to or less than 1%.

In contrast with this, the optical fiber 100 of the example 9 inaccordance with the present invention aims to implement the highnonlinearity as described above, and has a small structure with the coresection 101 having a diameter of about 1-2 μm by increasing the relativerefractive-index difference Δ to about 2-4% as illustrated in FIG. 19.Thus, the structure and object of the optical fiber 100 differ greatlyfrom those of the optical fiber 120 of the conventional example.

The patent document 3 proposes optical fiber 130 as shown in FIG. 21including three or six air holes 132 a formed in such a manner that theeffective refractive index difference between the core section 131 andthe cladding section 132 becomes greater than 5%. Since the opticalfiber 130 has both the core section 131 and cladding section 132composed of the same glass material (single glass), the core section 131at the center and the cladding section 132 outside the air holes 132 ahave the same refractive index, which is usually called a W-typestructure. However, the optical fiber 100 of the example 9 in accordancewith the present invention has the structure different from that of theoptical fiber 130 of the conventional example in the same manner asdescribed above.

EXAMPLE 10

As an example 10 in accordance with the present invention, it ispossible to form such optical fibers 140 and 150 as shown in FIGS. 22and 23 that have the air holes 102 a of the first cladding section 102formed in a multilayer (double layer) fashion in the radial direction ofthe first cladding section 102.

As a variation of the example 10 in accordance with the presentinvention, it is also possible to make the cross section geometry in theradial direction of the air holes 102 a in the first cladding section102 an ellipse or polygon.

EXAMPLE 11

As an example 11 in accordance with the present invention, it is alsopossible to make the equivalent refractive index of the first claddingsection 102 approximately equal to the refractive index of the secondcladding section 103 by filling the air holes 102 a in the firstcladding section 102 with a glass material with a refractive index lowerthan the refractive index of the tellurite glass constituting the secondcladding section 103 by Δn as shown in FIG. 24. In this case, as thetellurite glass, the glass as defined in claim 1 with the zerodispersion wavelength equal to or greater than 2 μm is used. Inparticular, among the glass composition ratios of the foregoing Table 1,using the composition ratios other than No. 1 or No. 6 are effective,and the glass doped with a rare-earth element as the glass material asdefined in claim 3 is further effective.

The optical fiber 160, the air holes 102 a of which are filled with theglass material rather than air, can improve the total mechanicalstrength as compared with the case where the air holes are filled air.At the same time, when fabricating the optical fiber by drawing from thepreform, it is possible to facilitate keeping a constant geometry of theair holes 102 a, and to reduce the scattering loss.

EXAMPLE 12

As an example of an example 12 in accordance with the present invention,it is also possible to achieve stronger optical confinement effect byforming an optical fiber 170 with the refractive index of the coresection 111 higher than the refractive index of the material of thefirst cladding section 102 as shown in FIG. 25, that is, by forming theaxial center without the air holes 102 a as a core section (center core)111 with a refractive index higher by an amount Δn. In this case, as thetellurite glass, the glass as defined in claim 1 with the zerodispersion wavelength equal to or greater than 2 μm is used. Inparticular, among the glass composition ratios of the foregoing Table 1,using the composition ratios other than No. 1 or No. 6 are effective,and the glass doped with a rare-earth element as the glass material asdefined in claim 3 is further effective.

As for the number and geometry of the air holes 102 a, they areappropriately determined so that the equivalent refractive index of thefirst cladding section 102 becomes approximately equal to the refractiveindex of the second cladding section 103.

In addition, it is not necessary for the optical fiber in accordancewith the present invention to satisfy the strict photonic bandgapcondition or the periodicity or uniformity condition as long as the ithas the total reflection structure based on the effective refractiveindex difference between the foregoing core section and claddingsection.

EXAMPLE 13

In the following examples 13-19 in accordance with the presentinvention, a fabrication method will be described of a glass preformwhen fabricating a fiber structure with the air holes by using thetellurite glass.

The example 13 in accordance with the present invention employs a moldhaving a plurality of portions convex on the inner wall as a mold to beused for casting the glass melt. The glass preform molded with the moldis inserted into a jacket tube composed of a hollow cylindricaltellurite glass to form an air hole section in a gap between the glasspreform and the jacket tube.

FIGS. 26A-26E show a fabrication method of the photonic crystal fiber ofthe example 13 in accordance with the present invention. Among the glasscomposition ratios in Table 1 with the thermal stability index Tx-Tgequal to or greater than 300° C., a glass melt 202 formed by melting theglass material of No. 19 composition is injected into a mold 201preheated to 300-400° C. (FIG. 26A). The mold 201 has four portionsconvex on the inner wall formed in such a manner that the injected glasspreform has a cross-shaped section. After injecting the glass melt,annealing at a temperature close to 300° C. is carried out for 10 hoursor more to fabricate the glass preform 203 (FIG. 26B). In this case,since the mold 201 is divided into four subdivisions to facilitatetaking out of the glass preform 203, it can prevent chipping or cracksof the glass preform 203. The hollow cylindrical jacket tube 204 isfabricated (FIG. 26C) by melting the glass materials in the same manneras described above, and by pouring the melt into a hollow cylindricalmold (not shown) which is preheated to 300-400° C., followed by arotational casting method that rotates the mold at a high speed withkeeping the mold in a horizontal position.

The glass preform 203 is inserted into the jacket tube 204, followed bybeing elongated (FIG. 26D). The elongated preform 205 has a preciselysymmetric cross section. A portion 206 of the elongated preform 205,which has a constant wire diameter, is cut therefrom, and is insertedinto another jacket tube (not shown) to be elongated again. The airholes are formed in the gap between the glass preform 203 and the jackettube. The portion 208 in which the holes are formed is pressed duringelongating and fiber drawing to carry out the fiber drawing underpressure in such a manner as to maintain or enlarge the air holes,thereby forming the air holes. Regulating the fiber drawing tension at50 g or greater in terms of the value before passing through a dice forcovering with resin, the fiber drawing process is performed to make theoutside diameter 110 μm (FIG. 26E), thereby fabricating the photoniccrystal fiber 207.

In the elongating process of the present example, the preform of 10-20mmφ is heated so that its viscosity becomes 10⁹-10¹⁰ P (poise) thatenables the elongating to 3-6 mmφ at the elongating weight of about 200g. On the other hand, to form the preform with a hole structure frombulk glass by the conventional extrusion process, it is necessary tosoften the bulk glass to the viscosity of about 10⁶ P (poise).Consequently, according to the method of the present example, theheating temperature is lower than that of the extrusion process. Thus,it can suppress the growing of the crystal nuclei, and is suitable forfabricating a low loss fiber.

FIG. 27A is a cross-sectional view showing the fabricated photoniccrystal fiber. The outside diameter of the photonic crystal fiber 207 is110 μm, and the inside diameter of the air holes is 26 μm. FIG. 27B isan enlarged view of the portion corresponding to the core fortransmitting light, and the core diameter is 2.6 μm. The cross sectionalarea A_(eff), at which the optical output becomes 1/e² of the peak, is3.54 μm², and the γ value (representing the nonlinearity and equal to2πn₂/λA_(eff) is 675 W⁻¹ km⁻¹.

The control of the core diameter and the inside diameter of the airholes is possible by varying the wall thickness of the jacket tube 204or increasing the number of elongating steps. The photonic crystal fiberof the present example has a loss of 60 dB/km at 1.55 μm, and the zerodispersion wavelength shifts from the material dispersion value 2.29 μmto 1.57 μm (see FIG. 28). Since the air holes are formed symmetrically,no polarization dependence occurs.

FIG. 29 illustrates the relationships between the core diameter and thezero dispersion wavelength of the photonic crystal fiber of the example13. Referring to FIG. 29, it is seen that the core diameter must becontrolled at 0.8-3.4 μm in order to control the zero dispersionwavelength at 1.2-1.7 μm. In addition, to make the zero dispersionwavelength 1.55 μm, the core diameter must be set at 2.45 μm.

FIG. 30 shows a configuration of the wavelength converter using thephotonic crystal fiber of the example 13. The wavelength converter haslight sources 301-332 for outputting 32 WDM signals at 100 GHz intervalsin the 1530-1560 nm wavelength band, and a light source 333 foroutputting 1565 nm pumping light. In addition, it has an AWG (ArrayedWaveguide Grating) 341 for multiplexing the outputs of the light sources301-332; an optical coupler 342 for coupling the multiplexed WDM signallight Es and the pumping light Ep; and a photonic crystal fiber 343 ofthe example 13 with the length of 50 m. With such a configuration, thewavelength converter collectively converts the wavelengths of the 32 WDMsignals, and outputs converted light Ec.

FIG. 31 shows an output spectrum of the wavelength converter. For thepumping light Ep with the power of 40 mW, the conversion efficiency is−15 dB, and the wavelength batch conversion of the bandwidth of 70 nmcan be carried out.

EXAMPLE 14

In the example 14 in accordance with the present invention, a fiber wasconstructed which had the same structure as the example 13 except fordoping Er by 5000 ppm by using the No. 14 sample as a glass composition.

A wavelength converter with the same configuration as that of FIG. 30 isfabricated using the photonic crystal fiber. The wavelength converteremploys light sources 301-332 for outputting 32 WDM signals at 100 GHzintervals in the 1530-1560 nm wavelength band, and a light source 333for outputting 1480 nm pumping light and 1565 nm pumping light. Using aphotonic crystal fiber 343 of the example 14 with a length of 15 m, thewavelength converter amplifies the signals and collectively converts thewavelengths at the same time. For the 1480 nm pumping light with powerof 50 mW, and the 1565 nm pumping light with power of 50 mW, it cancarry out the wavelength batch conversion whose conversion efficiency is5 dB and bandwidth is 70 nm.

Incidentally, applying the photonic crystal fiber of the present example14 with a length of 15 m to a nonlinear fiber loop mirror which will bedescribed later with reference to FIG. 44 makes it possible to carry outswitching of the signal light which undergoes high-speed modulation andhas a repetition frequency of 80 GHz and pulse width of 8 ps by gatelight with power of 10 mW.

FIG. 37 shows a parametric optical amplifier using a photonic crystalfiber of the present example 14. The parametric optical amplifierincludes a wavelength variable light source 1301, an isolator 1302, thephotonic crystal fiber 1303 of the present example 14 with a length of150 m, and an optical coupler 1304 which are connected in cascade. Theoptical coupler 1304 receives at its rear end the output of a lightsource 1305 with a wavelength of 1560 nm and pumping light power of 1.5W via an EDFA amplifier 1306.

FIG. 38 illustrates an output spectrum of the parametric opticalamplifier. The output spectrum, which is obtained as a result ofwavelength scanning measurement using signal light of −30 dBm, showsthat a gain equal to or greater than 20 dB is obtained over a 120 nmwavelength band from 1500 to 1620 nm.

EXAMPLE 15

The method of the example 15 in accordance with the present inventionfabricates a solid cylindrical glass block composed of tellurite glass,and then fabricates a glass preform including an air hole section bydrilling holes in the longitudinal direction of the glass block. In thismethod, the glass preform is inserted into a jacket tube composed ofhollow cylindrical tellurite glass, followed by fiber drawing.

FIGS. 32A-32C shows a fabrication method of the photonic crystal fiberof the example 15 in accordance with the present invention. Referring tothe foregoing Table 1, a glass melt formed by melting the glass materialof No. 15 composition with the thermal stability index Tx-Tg equal to orgreater than 300° C. is injected into a mold preheated to 300-400° C.After injecting the glass melt, annealing at a temperature close to 300°C. is carried out for 10 hours or more to fabricate the solidcylindrical glass block 601 (FIG. 32A). A plurality of holes are boredthrough the glass block 601 in its longitudinal direction with a 3 mmφdrill 602, thereby forming a glass preform 603 (FIG. 32B). The glasspreform 603 is elongated to 3 mmφ, and a portion 604 of the elongatedpreform at which wire diameter is constant is cut to fabricate thephotonic crystal fiber (FIG. 32C).

FIG. 33 is a cross-sectional view showing the photonic crystal fiberfabricated. The outside diameter of the photonic crystal fiber is 110μm, the air hole diameter d is 1.6 μm, the pitch A between the air holesis 2.3 μm and hence d/A=0.7. The MFD (Mode Field Diameter) is 3 μm, theloss of the fiber is 40 dB/km at 1.55 μm, and the zero dispersionwavelength is 1.55 μm.

Into the photonic crystal fiber with a length of 150 m, pulse pumpinglight is launched whose wavelength is 1.55 μm, pulse width is 0.5 ps,and peak power is 30 W. In this case, the photonic crystal fiber outputssupercontinuum light over a 1.7 μm bandwidth (0.7-2.4 μm) as illustratedin FIG. 34.

EXAMPLE 16

In the example 16 in accordance with the present invention, a mold whichis used for cast molding glass melt has a plurality of cylindricalrodlike pins disposed inside the mold from its bottom. Following thecast molding, the preheated pins are extracted quickly to form an airhole section.

FIGS. 35A-35B shows the fabrication method of the photonic crystal fiberof the example 16 in accordance with the present invention. Referring tothe foregoing Table 1, a glass melt 802 formed by melting the glassmaterial of No. 9 composition with the thermal stability index Tx-Tgequal to 180° C. is injected into molds 801 a and 801 b (a combinationof which is designated by a reference numeral 801 from now on) preheatedto 300-400° C. (FIG. 35A). The mold 801 has a jig including a pluralityof cylindrical rodlike pins 805 arranged on its internal base 804. Aftercasting the glass melt 802 in the mold 801, the pins 805 are extractedquickly, thereby fabricating the glass preform 803 having the air holesformed (FIG. 35B).

The photonic crystal fiber is fabricated by carrying out elongating andfiber drawing using the glass preform 803 in the same manner as theforegoing example 15. The photonic crystal fiber fabricated has the samecross section as that of FIG. 33: its outside diameter is 120 μm, airhole diameter d is 1.5 μm, pitch Δ between the air holes is 2.3 μm, andhence d/Δ=0.65. The MFD is 2.5 μm, the loss of the fiber is 65 dB/km at1.55 μm, and the zero dispersion wavelength is 1.55 μm.

Into the photonic crystal fiber with a length of 50 m, pulse pumpinglight is launched whose wavelength is 1.55 μm, pulse width is 0.5 ps,and peak power is 30 W. In this case, the pulses undergo soliton effect,and “soliton self phase shift” is observed which shifts the pulsespectrum toward the longer wavelength side as the pulses travel throughthe fiber.

FIG. 36 shows a wavelength variable pulse light source using thephotonic crystal fiber of the present example 16. The light source is awavelength variable pulse light source that utilizes the effect thatvaries the spectrum shift amount by varying the peak power of theincident pulse. The wavelength variable pulse light source includes apulse light source 901 modulated at 10 GHz, an optical amplifier 902, aphotonic crystal fiber 903 of the present example 16 with a length of 50m, and a programmable PLC (planer lightwave circuit) multi-demultiplexer904, which are connected in cascade.

In addition, an optical amplifier 905, and a photonic crystal fiber 906of the present example 16 with a length of 50 m are connected in cascadeto the output of the programmable PLC multi-demultiplexer 904. With sucha configuration, the wavelength variable pulse light source outputsoptical pulses with a wavelength variable range of 150 nm (1550-1700 nm)at a channel rate of 10-100 Gbit/s.

A wavelength converter with the same configuration as that of FIG. 30 isfabricated using the photonic crystal fiber. The wavelength converteremploys light sources 301-332 for outputting 32 WDM signals at 100 GHzintervals in the 1530-1560 nm wavelength band, and a light source 333for outputting 1480 nm pumping light and 1565 nm pumping light. Using aphotonic crystal fiber 343 of the example 16 with a length of 15 m, thewavelength converter amplifies the signals and collectively converts thewavelengths at the same time. For the 1480 nm pumping light with powerof 50 mW, and the 1565 nm pumping light with power of 50 mW, it cancarry out the wavelength batch conversion whose conversion efficiency is5 dB and bandwidth is 70 nm.

Incidentally, applying the photonic crystal fiber of the present example16 with a length of 15 m to the nonlinear fiber loop mirror which willbe described later with reference to FIG. 44 makes it possible to carryout switching of the signal light which undergoes high-speed modulationand has a repetition frequency of 80 GHz and pulse width of 8 ps by gatelight with power of 10 mW.

FIG. 37 shows a parametric optical amplifier using a photonic crystalfiber of the present example 16. The parametric optical amplifierincludes a wavelength variable light source 1301, an isolator 1302, aphotonic crystal fiber 1303 of the present example 16 with a length of150 m, and an optical coupler 1304 which are connected in cascade. Theoptical coupler 1304 receives at its rear end the output of a lightsource 1305 with a wavelength of 1560 nm and pumping light power of 1.5W via an EDFA amplifier 1306.

FIG. 38 illustrates an output spectrum of the parametric opticalamplifier. The output spectrum, which is obtained as a result ofwavelength scanning measurement using signal light of −30 dBm, showsthat a gain equal to or greater than 20 dB is obtained over a 120 nmwavelength band from 1500 to 1620 nm.

EXAMPLE 17

In the example 17 in accordance with the present invention, the photoniccrystal fiber has a core/cladding structure with a composition ofdifferent refractive indices rather than with a single composition oftellurite glass.

In addition, in the example 17 in accordance with the present invention,the mold used for casting the glass melt is processed in such a mannerthat its lower portion is conically enlarged. Using the mold, thecladding and core are injected sequentially, and a preform is used whichis formed by conically suction molding the core glass by the volumecontraction of the cladding glass. In this case, the upper portion ofthe mold has an inner wall with convex toward the inside, and the areasinto which the core glass is suctioned are small. Accordingly, tosuction the core effectively, optimization of the injection temperatureand the like is required.

FIGS. 39A-39B and FIGS. 40A-40B show a fabrication method of thephotonic crystal fiber of the example 17 in accordance with the presentinvention. As the core glass, the No. 18 composition of the foregoingTable 1 is used, and Tm is doped by 4000 ppm. As the cladding glass, theNo. 17 composition of the foregoing Table 1 is used. As for the mold1501, a plurality of portions are formed on the inner wall as in themold of FIG. 26A, and its lower portion is conically enlarged toward itsbottom (FIG. 39A). The mold 1501 is preheated at 300-400° C., the glassmelt 1502 of the cladding and the glass melt 1503 of the core aresequentially injected, and the glass preform 1504 is fabricated whosecore glass is conically suction molded by the volume contraction of thecladding glass (FIG. 39B). The suction length of the core glass is 15mm.

The photonic crystal fiber 1505 is fabricated by carrying out the sameelongating and fiber drawing process as that of the foregoing example 13using the glass preform 1504. FIG. 40A is a cross-sectional view showingthe photonic crystal fiber 1505. As for the photonic crystal fiber 1505,the outside diameter is 110 μm, the inside diameter of the air holes is35 μm, the cross-shaped central section is 2.4 μm and the core diameterdoped with Tm is 1.5 μm. The MFD is 2.9 μm, and the loss of the fiber is30 dB/km at 1.55 μm. Thus, introducing, as the core, glass componentsdifferent from those of the cladding, makes it possible to reduce theloss as compared with the case without having the above core/claddingstructure. The zero dispersion wavelength is 1.52 μm.

Splicing the photonic crystal fiber of the present example 17 to asilica fiber (with a relative refractive index difference of 4%, and MFDof 3 μm) using a commercially available fusion splicer enables thesplicing at a loss of 0.2 dB and a return loss equal to or less than −50dB. For the purpose of comparison, splicing the photonic crystal fiberwith the single composition of the foregoing example 13 and the silicafiber has a loss of 2 dB and a return loss of −19 dB because of thecollapse of the core geometry.

Consider the case where the photonic crystal fiber of the presentexample 17 with a length of 20 m is applied to the wavelength converterof FIG. 30. The WDM signal light Es multiplexed by the AWG 341 of thewavelength converter is a signal formed by multiplexing 32 WDM signalsin the wavelength band of 1480-1510 nm at 100 GHz intervals. The pumpinglight Ep consists of the 1410 nm pumping light used for exciting Tm, andthe 1520 nm pumping light used for both the wavelength conversion and Tmexcitation. The wavelength converter amplifies the signals andcollectively converts the wavelengths of the 32 WDM signals at the sametime, and outputs as the converted light Ec. In addition, the wavelengthconverter can carry out the wavelength batch conversion with theconversion efficiency of 5 dB and the bandwidth of 70 nm for the 1480 nmpumping light with power of 50 mW, and for the 1565 nm pumping lightwith power of 50 mW.

EXAMPLE 18

To facilitate the core suction as compared with the foregoing example17, the example 18 in accordance with the present invention has astructure that enables opening of a hole after injecting glass to thebottom of the conical section of the lower portion of the mold. Leakageof the glass from the hole brings about the synergistic effect with thecontraction of the glass. Drawing a vacuum in order to leak the glassout of the hole causes the synergistic effect with the contraction ofglass.

FIGS. 41A-41C show a fabrication method of the photonic crystal fiber ofthe example 18 in accordance with the present invention. As the coreglass, the No. 20 composition of the foregoing Table 1 is used, and asthe cladding glass, the No. 21 composition of the foregoing Table 1 isused. A mold 1601 has a plurality of portions convex on the inner wall,and is processed in such a manner that its lower portion is conicallyenlarged toward its bottom just as the mold shown in FIG. 39A (FIG.41A). In addition, the mold has such a geometry that includes a base1602 mounted on the bottom of the mold 1601, and that sliding a movablemember 1603 at the center of the base 1602 can form a through hole atthe bottom of the mold 1601 (FIG. 41B).

The mold 1601 is preheated at 300-400° C., and the base 1602 ispreheated at 350-450° C. independently. Then, the glass melt 1604 of thecladding and the glass melt 1605 of the core are injected successively(FIG. 41A). Because of the volume contraction of the cladding glass andthe hole opened at the bottom (FIG. 41B), the main portion of thecladding glass flows out so that a glass preform 1606 is obtained whichis formed by sucking the core glass (FIG. 41C). The suction length ofthe core is 25 mm.

The photonic crystal fiber 1505 is fabricated by carrying out the sameelongating and fiber drawing process as that of the foregoing example 13using the glass preform 1606 thus formed. The structure of the photoniccrystal fiber is the same as that of the foregoing FIG. 40A: the outsidediameter is 115 μm, the inside diameter of the air holes is 20 μm, thecross-shaped central section is 2.8 μm and the core diameter is 1.2 μm.The MFD is 2.5 μm, the loss of the fiber is 25 dB/km at 1.55 μm, and thezero dispersion wavelength is 1.55 μm.

FIG. 42 shows an optical Kerr shutter experimental system using thephotonic crystal fiber of the present example 18. The optical Kerrshutter experimental system includes a DFB-LD (distributedfeedback-laser diode) 1701 for outputting control light with awavelength of 1552 nm, a DFB-LD 1702 for outputting signal light with awavelength of 1535 nm, and an Er-doped fiber amplifier 1703 foramplifying the control light, and the control light and signal light arelaunched into a photonic crystal fiber 1704 of the present example 18with a length of 10 m in such a manner that their polarizationdirections make 45 degrees with each other. The signal light is branchedfrom the output of the photonic crystal fiber 1704, and is input to astreak camera 1706 via a polarizer 1705.

With such a configuration, when the control light is not launched into,the polarized wave of the signal light travels through the photoniccrystal fiber 1704 with a certain fixed direction, and is intercepted bythe polarizer 1705. On the other hand, when the control light islaunched into, because of the nonlinear refractive index effect of thephotonic crystal fiber 1704, the polarized components of the signallight change and transmit through the polarizer 1705. In this way, theoptical Kerr shutter experimental system can switch the signal lightpulse with a width of 8 ps.

EXAMPLE 19

To facilitate the suction of the core, the example 19 in accordance withthe present invention has a structure that opens the hole afterinjecting glass to the bottom of the conical section of the lowerportion of the mold, and draws to a vacuum in order to leak the glassout of the hole, thereby bringing about the synergistic effect with thecontraction of the glass.

In addition, during the fiber drawing under pressure carried out in sucha manner as to keep or enlarge the air holes formed in the preform, theexample 19 in accordance with the present invention facilitates thecontrol of the hole formation and hole diameter by making the tension inthe fiber drawing equal to or greater than 50 g.

FIG. 43A-FIG. 43C show a fabrication method of the photonic crystalfiber of the example 19 in accordance with the present invention. As thecore glass, the No. 13 composition of the foregoing Table 1 is used, andas the cladding glass, the No. 16 composition of the foregoing Table 1is used. A mold 1801 has a plurality of portions convex on the innerwall, and is processed in such a manner that its lower portion isconically enlarged toward its bottom just as the mold shown in FIG. 39A(FIG. 43A). In addition, the mold 1801 has such a geometry that includesa base 1802 mounted on the bottom of the mold 1801, and that sliding amovable member 1803 at the center of the base 1802 can form a hole atthe bottom of the mold 1801 (FIG. 43B). Using the hole enables vacuumdegassing from the bottom of the mold 1801.

The mold 1801 is preheated at 300-400° C., and the base 1802 ispreheated at 350-450° C. independently. Then, the glass melt 1804 of thecladding and the glass melt 1805 of the core are injected successively(FIG. 43A). Because of the volume contraction of the cladding glass andthe vacuum degassing from hole at the bottom (FIG. 43B), the mainportion of the cladding glass flows out so that a glass preform 1806 isobtained which is formed by sucking the core glass (FIG. 43C). Thesuction length of the core is 50 mm.

The photonic crystal fiber is fabricated by carrying out the sameelongating and fiber drawing process as that of the foregoing example 13using the glass preform 1806 thus formed. The structure of the photoniccrystal fiber is the same as that of the foregoing FIGS. 40A and 40B:the outside diameter of the fiber is 120 μm, the inside diameter of theair holes is 28 μm, the cross-shaped central section is 2.6 μm and thecore diameter is 1.3 μm. The MFD is 2.3 μm, the loss of the fiber is 28dB/km at 1.55 μm, and the zero dispersion wavelength is 1.56 μm.

FIG. 44 shows the nonlinear fiber loop mirror using a photonic crystalfiber of the present example 19. The nonlinear fiber loop mirrorincludes an optical coupler 1901 into which gate light is launched, aphotonic crystal fiber 1902 of the present example 19 with a length of15 m, an optical coupler 1903 for outputting the gate light, and anoptical coupler 1904 for inputting and outputting signal light, whichare connected in cascade to form a loop.

The signal light is bifurcated by the optical coupler 1904, and the twosignal lights travel through the photonic crystal fiber 1902 in forwardand reverse directions. The signal lights are input to the opticalcoupler 1904 again, interfere with each other, and are output. In thiscase, switching is carried out by controlling the phase changes of thesignal light in the photonic crystal fiber 1902 in response to the gatelight input to the optical coupler 1901. The gate light with power of200 mW enables the switching of the signal light which undergoeshigh-speed modulation, and has the repetition frequency of 80 GHz andpulse width of 8 ps.

Using the same fabrication method of the example 13 shown in FIGS.26A-26E, up to the elongating process is carried out after inserting theNo. 11 composition of the foregoing Table 1 into the jacket tube. In thepresent example 19, maintaining the pressure to the air holes at a fixedvalue, the fiber drawing tension is adjusted at 50 g or greater in termsof the value before passing through a dice for covering with resin. Asillustrated in FIGS. 27A-27B, the outside diameter of the photoniccrystal fiber is 110 μm. As for a fiber #1 (1000 m) fabricated by thepresent method and a fiber #2 (1000 m) processed at the fiber drawingtension of 30 g, the stability of the inside diameters of the air holesin the longitudinal direction is compared.

The fiber #1 has an error of ±5 μm for the design value of 26 μm of theinside diameter of the air holes. The actually usable portions within 26μm±1 μm are 70% of the total length, and even the shortest portion has50 m or more length. On the other hand, the fiber #2 has an error of ±20μm for the design value of 26 μm of the inside diameter of the airholes. The actually usable portions within 26 μm±1 μm are 20% of thetotal length, and only a few portions have 50 m or more length.

It is found from the foregoing comparison that it is important to setthe fiber drawing tension at 50 g or more in terms of the value beforepassing through the dice for covering with resin in the fiber drawingprocess carried out with matching the size of the air holes to thedesign value and maintaining it. The setting is also important for thefiber drawing process of other photonic crystal fibers. The cross-shapedcenter of the fiber #1 is 2.6 μm. The MFD is 2.4 μm, the loss of thefiber is 24 dB/km at 1.55 μm, and the zero dispersion wavelength is 1.56μm.

FIG. 45 shows a clock reproduction apparatus using a photonic crystalfiber of an example 19 in accordance with the present invention. Theclock reproduction apparatus 2003 of a WDM transmission system receiveswith a clock reproduction section 2201 a single wavelength signalselected by a wavelength selective filter 2002, to which the WDM signaltransmitted from a transmitter 2001 is input, and extracts an RF clocksignal. A mode-locked fiber laser in the clock reproduction section 2201reproduces an optical pulse from the extracted clock signal. An EDFA2204 amplifies the optical pulse train, and supplies it to a photoniccrystal fiber 2203 of the present example 19 with a length of 30 m. Thephotonic crystal fiber 2203 generates supercontinuum light over a 100 nmbandwidth from 1.5 to 1.6 μm, and supplies it to an AWG 2204. The AWG2204 carries out filtering to restore the clock pulse signals for thechannels passing through the wavelength division multiplexing from thesingle channel clock reproduction.

The clock pulse signal of any one of the channels is launched into anonlinear loop mirror 2004 using a photonic crystal fiber of the presentexample 19 with a length of 50 m. Supplying the nonlinear loop mirror2004 with the channel corresponding to the WDM signal transmitted fromthe transmitter 2001 as the gate light makes it possible to implement anoptical 3R reproduction that restores degraded signal quality.

Although the foregoing examples 13-19 have a plurality of portionsformed in such a manner that they are convex on the inner wall of themold, and four air holes formed, the number of the air holes is notlimited to that number. In addition, optical devices that use thepresent fiber are not limited to the foregoing examples 13-19, but areoptical devices that employ the present fiber as a highly nonlinearfiber.

As described above, the fabrication method of the optical fibers of theexamples 13-19 in accordance with the present invention forms the glasspreform by the cast molding or compression molding. In either of thesemolding methods, since the duration of heating the glass preform isshorter than that of the conventional extrusion process, thecrystallization in the glass can be suppressed, and low loss opticalfibers can be fabricated.

EXAMPLE 20

The following examples 20-31 in accordance with the present inventiondisclose a method that disposes a plurality of air holes near the centerof a tellurite glass optical fiber, and controls the dispersioncharacteristics of the fiber by the size of the region surrounded by theair holes.

FIG. 46 shows a cross sectional view of an optical fiber of the example20 in accordance with the present invention. Tellurite glass 2101 whichis inserted into a jacket tube 2104 and has a zero-material dispersionwavelength of 2.08 μm has four air holes 2103 a-2103 d (designated by ageneric number 2103 from now on). The air holes 2103 are filled with airand their refractive index is approximately one. The portion surroundedby the four air holes 2103 is a region 2102 to become a core fortransmitting light. The outside diameter of the tellurite glass 2101 is100 μm, the inside diameter of the air holes 2103 is 40 μm, and the corediameter is 4.5 μm. The cross sectional area A_(eff), at which theoptical output becomes 1/e² of the peak, is 4.1 μm², and the γ value is590 W⁻¹ km⁻¹.

The fabrication process of the photonic crystal fiber of the presentexample 20 is the same as the fabrication process of FIGS. 26A-26E.Although it will be a duplicate description, a fabrication method of thephotonic crystal fiber of the present example 20 will be described forconfirmation with reference to FIGS. 26A-26E. The glass melt 202 formedby melting tellurite glass materials is injected into the mold 201preheated at 300-400° C. (FIG. 26A). The mold 201 has four portionsconvex on the inner wall formed in such a manner that the injected glasspreform has a cross-shaped section. After injecting the glass melt,annealing at a temperature close to 300° C. is carried out for 10 hoursor more to fabricate the glass preform 203 (FIG. 26B). In this case,since the mold 201 is divided into four subdivisions to facilitatetaking out of the glass preform 203, it can prevent chipping or cracksof the glass preform 203. The hollow cylindrical jacket tube 204 isfabricated (FIG. 26C) by melting the glass materials in the same manneras described above, and by pouring the melt into a hollow cylindricalmold (not shown) which is preheated to 300-400° C., followed by arotational casting method that rotates the mold at high speed withkeeping the mold in a horizontal position.

The glass preform 203 is inserted into the jacket tube 204, followed bybeing elongated (FIG. 26D). The elongated preform 205 has a preciselysymmetric cross section. A portion 206 of the elongated preform 205,which has a constant wire diameter, is cut therefrom, and is insertedinto another jacket tube (not shown) to be elongated again. The airholes are formed in the gap between the glass preform 203 and the jackettube. The portion 208 in which the holes are formed is pressed duringelongating and fiber drawing to carry out the fiber drawing underpressure in such a manner as to maintain or enlarge the air holes,thereby forming the air holes. Regulating the fiber drawing tension at50 g or greater in terms of the value before passing through a dice forcovering with resin, the fiber drawing process is performed to make theoutside diameter 105 μm (FIG. 26E), thereby fabricating the photoniccrystal fiber 207.

In the elongating process of the present example 20, the preform of10-20 mmφ is heated so that its viscosity becomes 10⁹-10¹⁰ P (poise)that enables the elongating to 3-6 mmφ at the elongating weight of about200 g. On the other hand, to form the preform with a hole structure frombulk glass by the conventional extrusion process, it is necessary tosoften the bulk glass to the viscosity of about 10⁶ P (poise).Consequently, according to the method of the present example, theheating temperature is lower than that of the conventional extrusionprocess. Thus, it can suppress the growing of the crystal nuclei, and issuitable for fabricating a low loss fiber.

FIG. 47 illustrates the opto-electric field distribution of the opticalfiber of the present example 20. The opto-electric field distribution isobtained using the calculus of finite difference method, one of thenumerical calculations. Each one of the contours shows every 10%difference in the electric field. It is found from the calculationresults that the optical fiber of the example 20 confines light withinthe core region 2102 at the center, and that the light propagatesthrough the core. Observing the near field pattern (NFP) and far fieldpattern (FFP) after cutting and polishing the optical fiber makes itpossible to confirm that the light is confined in the fiber centralsection, and the single mode is achieved.

FIG. 48 illustrates the wavelength dispersion of the optical fiber ofthe present example 20. The zero dispersion wavelength λ₀ of the opticalfiber of the example 20 is 1.56 μm.

EXAMPLE 21

FIG. 49 shows an optical fiber of the example 21 in accordance with thepresent invention. Tellurite glass 2301 with the No. 15 composition ofthe foregoing Table 1, which is inserted into a jacket tube 2304, hasfour air holes 2303 a-2303 d (designated by a generic number 2303)formed therein, and the air holes 2303 are filled with air so that therefractive index is approximately equal to one. The portion surroundedby the four air holes 2303 is a region 2302 to become a core fortransmitting light. In the region 2302, tellurite glass 2305 is buriedwhich is obtained by changing the composition of tellurite glass, andwhich has a zero-material dispersion wavelength of 2.1 μm and arefractive index higher than that of the tellurite glass 2301 by 1.1% interms of a relative refractive-index difference. In the present example21, the optical fiber was fabricated by a capillary method. The outsidediameter of the tellurite glass 2301 is 110 μm, the inside diameter ofthe air holes 2303 is 35 μm, and the core diameter is 3.0 μm. The crosssectional area A_(eff), at which the optical output becomes 1/e² of thepeak, is 2.6 μm², and the γ value is 940 W⁻¹ km⁻¹.

FIG. 50 illustrates the opto-electric field distribution of the opticalfiber of the present example 21. The opto-electric field distribution isobtained using the calculus of finite difference method, one of thenumerical calculations. Each one of the contours shows every 10%difference in the electric field. It is found from the calculationresults that the optical fiber of the example 21 confines light withinthe core region 2302 at the center, and that the light propagatesthrough the core. Observing the near field pattern (NFP) and far fieldpattern (FFP) after cutting and polishing the optical fiber makes itpossible to confirm that the light is confined in the fiber centralsection, and the single mode is achieved.

FIG. 51 illustrates the wavelength dispersion of the optical fiber ofthe present example 21. The zero dispersion wavelength λ₀ of the opticalfiber of the example 21 is 1.30 μm.

EXAMPLE 22

FIG. 52 shows an optical fiber of the example 22 in accordance with thepresent invention. Tellurite glass 2401 with the No. 18 composition ofthe foregoing Table 1, which is inserted into a jacket tube 2404, hasfour air holes 2403 a-2403 d (designated by a generic number 2403)formed therein, and the air holes 2403 are filled with air so that therefractive index is approximately equal to one. The portion surroundedby the four air holes 2403 is a region 2402 to become a core fortransmitting light. In the region 2402, tellurite glass 2405 is buriedwhich is obtained by changing the composition of tellurite glass, andwhich has a zero-material dispersion wavelength of 2.05 μm and arefractive index lower than that of the tellurite glass 2401 by 2.2% interms of a relative refractive-index difference. In the present example22, the optical fiber was fabricated by a capillary method. The outsidediameter of the tellurite glass 2401 is 90 μm, the inside diameter ofthe air holes 2403 is 45 μm, and the core diameter is 2.7 μm. The crosssectional area A_(eff), at which the optical output becomes 1/e² of thepeak, is 2.5 μm², and the γ value is 930 W⁻¹ km⁻¹.

FIG. 53 illustrates the opto-electric field distribution of the opticalfiber of the present example 22. The opto-electric field distribution isobtained using the calculus of finite difference method, one of thenumerical calculations. Each one of the contours shows every 10%difference in the electric field. It is found from the calculationresults that the optical fiber of the example 22 confines light withinthe core region 2402 at the center, and that the light propagatesthrough the core. Observing the near field pattern (NFP) and far fieldpattern (FFP) after cutting and polishing the optical fiber makes itpossible to confirm that the light is confined in the fiber centralsection, and the single mode is achieved.

FIG. 54 illustrates the wavelength dispersion of the optical fiber ofthe present example 22. The zero dispersion wavelength λ₀ of the opticalfiber of the example 22 is 1.52 μm.

EXAMPLE 23

FIG. 55 shows an optical fiber of the example 23 in accordance with thepresent invention. Tellurite glass 2501 with the No. 17 composition ofthe foregoing Table 1, which is inserted into a jacket tube 2504, hasfour air holes 2503 a-2503 d (designated by a generic number 2503)formed therein, and the air holes 2503 are filled with air so that therefractive index is approximately equal to one. The portion surroundedby the four air holes 2503 is a region 2502 to become a core fortransmitting light. In the region 2502, a central air hole 2505 isformed. In the present example 23, the optical fiber was fabricated by acapillary method. The outside diameter of the tellurite glass 2501 is105 μm, the inside diameter of the air holes 2503 is 40 μm, and the corediameter is 3.1 μm. The cross sectional area A_(eff), at which theoptical output becomes 1/e² of the peak, is 2.8 μm², and the γ value is810 W⁻¹ km⁻¹.

FIG. 56 illustrates the opto-electric field distribution of the opticalfiber of the present example 23. The opto-electric field distribution isobtained using the calculus of finite difference method, one of thenumerical calculations. Each one of the contours shows every 10%difference in the electric field. It is found from the calculationresults that the optical fiber of the example 23 confines light withinthe core region 2502 at the center, and that the light propagatesthrough the core. Observing the near field pattern (NFP) and far fieldpattern (FFP) after cutting and polishing the optical fiber makes itpossible to confirm that the light is confined in the fiber centralsection, and the single mode is achieved.

FIG. 57 illustrates the wavelength dispersion of the optical fiber ofthe present example 23. The zero dispersion wavelength λ₀ of the opticalfiber of the example 23 is 1.41 μm.

EXAMPLE 24

FIG. 58 shows an optical fiber of the example 24 in accordance with thepresent invention. Tellurite glass 2601 with the No. 14 composition ofthe foregoing Table 1, which is inserted into a jacket tube 2604, hasthree air holes 2603 a-2603 c (designated by a generic number 2603)formed therein, and the air holes 2603 are filled with air so that therefractive index is approximately equal to one. The portion surroundedby the three air holes 2603 is a region 2602 to become a core fortransmitting light. In the present example 24, the optical fiber wasfabricated by the extrusion process. The outside diameter of thetellurite glass 2601 is 110 μm, the inside diameter of the air holes2603 is 40 μm, and the core diameter is 5.5 μm. The cross sectional areaA_(eff), at which the optical output becomes 1/e² of the peak, is 4.5μm², and the γ value is 520 W⁻¹ km⁻¹.

Observing the near field pattern (NFP) and far field pattern (FFP) aftercutting and polishing the optical fiber makes it possible to confirmthat the light is confined in the fiber central section, and the singlemode is achieved.

FIG. 59 illustrates the wavelength dispersion of the optical fiber ofthe present example 24. The zero dispersion wavelength λ₀ of the opticalfiber of the example 24 is 1.65 μm.

EXAMPLE 25

FIG. 60 shows an optical fiber of the example 25 in accordance with thepresent invention. Tellurite glass 2701 with the No. 16 composition ofthe foregoing Table 1, which is inserted into a jacket tube 2704, hasfour air holes 2703 a-2703 d (designated by a generic number 2703)formed therein, and the air holes 2703 are filled with air so that therefractive index is approximately equal to one. The portion surroundedby the four air holes 2703 is a region 2702 to become a core fortransmitting light. In the present example 25, the optical fiber wasfabricated by the extrusion process. The outside diameter of thetellurite glass 2701 is 110 μm, the inside diameter of the air holes2703 is 40 μm, and the core diameter is 2.2 μm. The cross sectional areaA_(eff), at which the optical output becomes 1/e² of the peak, is 2.0μm², and the γ value is 1200 W⁻¹ km⁻¹.

Observing the near field pattern (NFP) and far field pattern (FFP) aftercutting and polishing the optical fiber makes it possible to confirmthat the light is confined in the fiber central section, and the singlemode is achieved.

FIG. 61 illustrates the wavelength dispersion of the optical fiber ofthe present example 25. The zero dispersion wavelength λ₀ of the opticalfiber of the example 25 is 1.22 μm.

EXAMPLE 26

FIG. 62 shows an optical fiber of the example 26 in accordance with thepresent invention. Tellurite glass 2801 with the No. 18 composition ofthe foregoing Table 1, which is inserted into a jacket tube 2804, hasfive air holes 2803 a-2803 e (designated by a generic number 2803)formed therein, and the air holes 2803 are filled with air so that therefractive index is approximately equal to one. The portion surroundedby the five air holes 2803 is a region 2802 to become a core fortransmitting light. In the region 2802, tellurite glass 2805 is buriedwhich is obtained by changing the composition of tellurite glass, andwhich has a zero-material dispersion wavelength of 2.1 μm and arefractive index higher than that of the tellurite glass 2801 by 1.1% interms of a relative refractive-index difference. In the present example26, the optical fiber was fabricated by the extrusion process. Theoutside diameter of the tellurite glass 2801 is 110 μm, the insidediameter of the air holes 2803 is 40 μm, and the core diameter is 4.1μm. The cross sectional area A_(eff), at which the optical outputbecomes 1/e² of the peak, is 3.5 μm², and the γ value is 680 W⁻¹ km⁻¹.

Observing the near field pattern (NFP) and far field pattern (FFP) aftercutting and polishing the optical fiber makes it possible to confirmthat the light is confined in the fiber central section, and the singlemode is achieved.

FIG. 63 illustrates the wavelength dispersion of the optical fiber ofthe present example 26. The zero dispersion wavelength λ₀ of the opticalfiber of the example 26 is 1.61 μm.

EXAMPLE 27

FIG. 64 shows an optical fiber of the example 27 in accordance with thepresent invention. Tellurite glass 2901 with the No. 12 composition ofthe foregoing Table 1, which is inserted into a jacket tube 2904, hassix air holes 2903 a-2903 f (designated by a generic number 2903) formedtherein, and the air holes 2903 are filled with air so that therefractive index is approximately equal to one. The portion surroundedby the six air holes 2903 is a region 2902 to become a core fortransmitting light. In the region 2902, tellurite glass 2905 is buriedwhich is obtained by changing the composition of tellurite glass, andwhich has a zero-material dispersion wavelength of 2.15 μm and arefractive index lower than that of the tellurite glass 2901 by 1.1% interms of a relative refractive-index difference. In the present example27, the optical fiber was fabricated by the extrusion process. Theoutside diameter of the tellurite glass 2901 is 110 μm, the insidediameter of the air holes 2903 is 40 μm, and the core diameter is 3.5μm. The diameter of the tellurite glass 2905 is 1.5 μm. The crosssectional area A_(eff), at which the optical output becomes 1/e² of thepeak, is 3.4 μm², and the γ value is 670 W⁻¹ km⁻¹.

Observing the near field pattern (NFP) and far field pattern (FFP) aftercutting and polishing the optical fiber makes it possible to confirmthat the light is confined in the fiber central section, and the singlemode is achieved.

FIG. 65 illustrates the wavelength dispersion of the optical fiber ofthe present example 27. The zero dispersion wavelength λ₀ of the opticalfiber of the example 27 is 1.70 μm.

EXAMPLE 28

FIG. 66 shows an optical fiber of the example 28 in accordance with thepresent invention. Tellurite glass 3001 with the No. 10 composition ofthe foregoing Table 1, which is inserted into a jacket tube 3004, hasthree air holes 3003 a-3003 c (designated by a generic number 3003)formed therein, and the air holes 3003 are filled with air so that therefractive index is approximately equal to one. The portion surroundedby the three air holes 3003 is a region 3002 to become a core fortransmitting light.

FIG. 67 is an enlarged view of the region to become the core of theoptical fiber of FIG. 66. In the present example 28, the optical fiberwas fabricated by ultrasonic drilling. The outside diameter of thetellurite glass 3001 is 100 μm, the inside diameter of the air holes3003 is 35 μm, and the core diameter a is 5.5 μm. The cross sectionalarea A_(eff), at which the optical output becomes 1/e² of the peak, is3.0 μm², and the γ value is 780 W⁻¹ km⁻¹.

Observing the near field pattern (NFP) and far field pattern (FFP) aftercutting and polishing the optical fiber makes it possible to confirmthat the light is confined in the fiber central section, and the singlemode is achieved.

FIG. 68 illustrates the relationships between the zero dispersionwavelength and core size of the optical fiber obtained in the presentexample 28. To set the zero dispersion wavelength in the 1.2 μm-1.7 μmband, it is found that the size of the region to become the core whichis surrounded by the air holes and confines the light, that is, the corediameter a, must be controlled in a range of 0.6 μm-6.5 μm.

EXAMPLE 29

FIG. 69 shows an optical fiber of the example 29 in accordance with thepresent invention. Tellurite glass 3101 with the No. 11 composition ofthe foregoing Table 1, which is inserted into a jacket tube 3104, hasfour air holes 3103 a-3103 d (designated by a generic number 3103)formed therein, and the air holes 3103 are filled with air so that therefractive index is approximately equal to one. The portion surroundedby the four air holes 3103 is a region 3102 to become a core fortransmitting light.

FIG. 70 is an enlarged view of the region to become the core of theoptical fiber of FIG. 69. In the present example 29, the optical fiberwas fabricated by ultrasonic drilling. The outside diameter of thetellurite glass 3101 is 125 μm, the inside diameter of the air holes3103 is 50 μm, and the core diameter is 3.5 μm. The cross sectional areaA_(eff), at which the optical output becomes 1/e² of the peak, is 3.2μm², and the γ value is 770 W⁻¹ km⁻¹.

Observing the near field pattern (NFP) and far field pattern (FFP) aftercutting and polishing the optical fiber makes it possible to confirmthat the light is confined in the fiber central section, and the singlemode is achieved.

FIG. 71 illustrates the relationships between the zero dispersionwavelength and core size obtained in the present example 29. To set thezero dispersion wavelength in the 1.2 μm-1.7 μm band, it is found thatthe size of the region to become the core which is surrounded by the airholes and confines the light, that is, the core diameter a, must becontrolled in a range of 0.6 μm-5.0 μm.

EXAMPLE 30

FIG. 72 shows an optical fiber of the example 30 in accordance with thepresent invention. Tellurite glass 3201 with the No. 17 composition ofthe foregoing Table 1, which is inserted into a jacket tube 3204, hasfive air holes 3203 a-3203 e (designated by a generic number 3203)formed therein, and the air holes 3203 are filled with air so that therefractive index is approximately equal to one. The portion surroundedby the five air holes 3203 is a region 3202 to become a core fortransmitting light.

FIG. 73 is an enlarged view of the region to become the core of theoptical fiber of FIG. 72. In the region 3202, tellurite glass 3205 isburied which is obtained by changing the composition of tellurite glass,and which has a zero-material dispersion wavelength of 2.2 μm and arefractive index higher than that of the tellurite glass 3201 by 1.1% interms of a relative refractive-index difference. In the present example30, the optical fiber was fabricated by the extrusion process. Theoutside diameter of the tellurite glass 3201 is 80 μm, the insidediameter of the air holes 3203 is 35 μm, and the core diameter is 3.9μm. The diameter of the tellurite glass 3205 is 1.0 μm. The crosssectional area A_(eff), at which the optical output becomes 1/e² of thepeak, is 3.4 μm², and the γ value is 690 W⁻¹ km⁻¹.

Observing the near field pattern (NFP) and far field pattern (FFP) aftercutting and polishing the optical fiber makes it possible to confirmthat the light is confined in the fiber central section, and the singlemode is achieved.

FIG. 74 illustrates the relationships between the zero dispersionwavelength and core size obtained in the present example 30. To set thezero dispersion wavelength in the 1.2 μm-1.7 μm band, it is found thatthe size of the region to become the core which is surrounded by the airholes and confines the light, that is, the core diameter a, must becontrolled in a range of 0.4 μm-5.0 μm.

EXAMPLE 31

FIG. 75 shows an optical fiber of the example 31 in accordance with thepresent invention. Tellurite glass 3301 with the No. 17 composition ofthe foregoing Table 1, which is inserted into a jacket tube 3304, hassix air holes 3303 a-3303 f (designated by a generic number 3303) formedtherein, and the air holes 3303 are filled with air so that therefractive index is approximately equal to one. The portion surroundedby the six air holes 3303 is a region 3302 to become a core fortransmitting light.

FIG. 76 is an enlarged view of the region to become the core of theoptical fiber of FIG. 75. In the region 3302, tellurite glass 3305 isburied which is obtained by changing the composition of tellurite glass,and which has a zero-material dispersion wavelength of 2.3 μm and arefractive index lower than that of the tellurite glass 3301 by 1.5% interms of a relative refractive-index difference. In the present example31, the optical fiber was fabricated by the extrusion process. Theoutside diameter of the tellurite glass 3301 is 95 μm, the insidediameter of the air holes 3303 is 50 μm, and the core diameter is 3.0μm. The diameter of the tellurite glass 3305 is 1.5 μm. The crosssectional area A_(eff), at which the optical output becomes 1/e² of thepeak, is 3.5 μm², and the γ value is 680 W⁻¹ km⁻¹.

Observing the near field pattern (NFP) and far field pattern (FFP) aftercutting and polishing the optical fiber makes it possible to confirmthat the light is confined in the fiber central section, and the singlemode is achieved.

FIG. 77 illustrates the relationships between the zero dispersionwavelength and core size obtained in the present example 31. To set thezero dispersion wavelength in the 1.2 μm-1.7 μm band, it is found thatthe size of the region to become the core which is surrounded by the airholes and confines the light, that is, the core diameter a, must becontrolled in a range of 0.3 μm-4.0 μm.

6. INDUSTRIAL APPLICABILITY

The optical fibers in accordance with the present invention, and thenonlinear devices formed by its fabrication method are effective forenhancing the performance, increasing the capacity and reducing the costof the optical communication systems, and hence contribute to theimprovement and cost reduction of the service using the systems, therebybeing very useful for the optical communication industry.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

1. An optical fiber composed of tellurite glass with a material zero-dispersion wavelength equal to or greater than 2 μm the optical fiber comprising: a core region; a first cladding section that is formed in such a manner as to enclose said core region, and has a plurality of air holes in a circumferential direction of said core region and along an axial direction of said core region; and a second cladding section that is formed in such a manner as to enclose said first cladding section, and has a refractive index approximately equal to an equivalent refractive index of said first cladding section; and wherein a relative refractive-index difference between said core region and said first cladding section is equal to or greater than 2%, thereby controlling a zero dispersion wavelength at a 1.55 μm band which is an optical telecommunication window.
 2. An optical fiber composed of tellurite glass with a material zero-dispersion wavelength equal to or greater than 2 μm and having a composition of TeO₂—Bi₂O₃-LO-M₂O—N₂O₃-O₂O₅, where L is at least one of Zn, Ba and Mg, M is at least one alkaline element selected from Li, Na, K, Rb and Cs, N is at least one of B, La, Ga, Al and Y, and Q is at least one of P and Nb, and components of said tellurite glass are 50<TeO₂<90 (mol %), 1<Bi₂O₃<30 (mol %), and 1<LO+M₂O+N₂O₃+Q₂O₅<50 (mol %), wherein said optical fiber comprises: a core region; a first cladding section that is formed in such a manner as to enclose said core region, and has a plurality of air holes in a circumferential direction of said core region and along an axial direction of said core region; and a second cladding section that is formed in such a manner as to enclose said first cladding section, and has a refractive index approximately equal to an equivalent refractive index of said first cladding section; and wherein a relative refractive-index difference between said core region and said first cladding section is equal to or greater than 2%, thereby controlling a zero dispersion wavelength at a 1.55 μm band which is an optical telecommunication window.
 3. An optical fiber composed of tellurite glass with a material zero-dispersion wavelength equal to or greater than 2 μm and having a composition of TeO₂—Bi₂O₃-LO-M₂O—N₂O₃-O₂O₅, where L is at least one of Zn, Ba and Mg, M is at least one alkaline element selected from Li, Na, K, Rb and Cs, N is at least one of B, La, Ga, Al and Y, and Q is at least one of P and Nb, and components of said tellurite glass are 50<TeO₂<90 (mol %), 1<Bi₂O₃<30 (mol %), and 1<LO+M₂O+N₂O₃+Q₂O₅<50 (mol %), wherein said tellurite material glass is doped with at least one type of rare-earth ions selected from Ce³⁺, Pr³⁺, Nd³⁺, Pm³⁺, Sm³⁺, Eu³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺ and Yb³⁺, and wherein said optical fiber comprises: a core region; a first cladding section that is formed in such a manner as to enclose said core region, and has a plurality of air holes in a circumferential direction of said core region and along an axial direction of said core region; and a second cladding section that is formed in such a manner as to enclose said first cladding section, and has a refractive index approximately equal to an equivalent refractive index of said first cladding section; and wherein a relative refractive-index difference between said core region and said first cladding section is equal to or greater than 2%, thereby controlling a zero dispersion wavelength at a 1.55 μm band which is an optical telecommunication window.
 4. The optical fiber as claimed in claim 1, wherein said air holes of said first cladding section are formed at fixed intervals along the circumferential direction of said core region.
 5. The optical fiber as claimed in claim 2, wherein said air holes of said first cladding section are formed at fixed intervals along the circumferential direction of said core region.
 6. The optical fiber as claimed in claim 3, wherein said air holes of said first cladding section are formed at fixed intervals along the circumferential direction of said core region.
 7. The optical fiber as claimed in claim 1, wherein said air holes of said first cladding section are formed in a multilayer fashion in a radial direction of said first cladding section.
 8. The optical fiber as claimed in claim 2, wherein said air holes of said first cladding section are formed in a multilayer fashion in a radial direction of said first cladding section.
 9. The optical fiber as claimed in claim 3, wherein said air holes of said first cladding section are formed in a multilayer fashion in a radial direction of said first cladding section.
 10. The optical fiber as claimed in claim 1, wherein said air holes of said first cladding section are filled with a material having a refractive index lower than a refractive index of said second cladding section.
 11. The optical fiber as claimed in claim 2, wherein said air holes of said first cladding section are filled with a material having a refractive index lower than a refractive index of said second cladding section.
 12. The optical fiber as claimed in claim 3, wherein said air holes of said first cladding section are filled with a material having a refractive index lower than a refractive index of said second cladding section.
 13. The optical fiber as claimed in claim 1, wherein said core region has a refractive index higher than a refractive index of a material of said first cladding section.
 14. The optical fiber as claimed in claim 2, wherein said core region has a refractive index higher than a refractive index of a material of said first cladding section.
 15. The optical fiber as claimed in claim 3, wherein said core region has a refractive index higher than a refractive index of a material of said first cladding section.
 16. The optical fiber as claimed in claim 1, wherein a central section to become said core has tellurite glass, a refractive index of which differs from the refractive index of said tellurite glass, embedded in said central section.
 17. The optical fiber as claimed in claim 2, wherein a central section to become said core has tellurite glass, a refractive index of which differs from the refractive index of said tellurite glass, embedded in said central section.
 18. The optical fiber as claimed in claim 3, wherein a central section to become said core has tellurite glass, a refractive index of which differs from the refractive index of said tellurite glass, embedded in said central section. 